diff --git a/pythonfmu/__init__.py b/pythonfmu/__init__.py
index 010886d..a5a976f 100644
--- a/pythonfmu/__init__.py
+++ b/pythonfmu/__init__.py
@@ -2,5 +2,6 @@
 from .builder import FmuBuilder
 from .enums import Fmi2Causality, Fmi2Initial, Fmi2Variability
 from .fmi2slave import Fmi2Slave
-from .variables import Boolean, Integer, Real, String
+from .variables import Boolean, Integer, Real, String, Enumeration
+from .type import Real as TypeReal, Integer as TypeInteger, Item, Enumeration as TypeEnum
 from .default_experiment import DefaultExperiment
diff --git a/pythonfmu/fmi2slave.py b/pythonfmu/fmi2slave.py
index 1599142..7ba9831 100644
--- a/pythonfmu/fmi2slave.py
+++ b/pythonfmu/fmi2slave.py
@@ -12,7 +12,8 @@
 from .default_experiment import DefaultExperiment
 from ._version import __version__ as VERSION
 from .enums import Fmi2Type, Fmi2Status, Fmi2Causality, Fmi2Initial, Fmi2Variability
-from .variables import Boolean, Integer, Real, ScalarVariable, String
+from .variables import Boolean, Integer, Real, ScalarVariable, String, Enumeration
+from .type import SimpleType, Real as TypeReal, Integer as TypeInteger, Item, Enumeration as TypeEnum
 
 ModelOptions = namedtuple("ModelOptions", ["name", "value", "cli"])
 
@@ -40,6 +41,7 @@ class Fmi2Slave(ABC):
 
     def __init__(self, **kwargs):
         self.vars = OrderedDict()
+        self.types = []
         self.instance_name = kwargs["instance_name"]
         self.resources = kwargs.get("resources", None)
         self.visible = kwargs.get("visible", False)
@@ -92,6 +94,11 @@ def to_xml(self, model_options: Dict[str, str] = dict()) -> Element:
 
         SubElement(root, "CoSimulation", attrib=options)
 
+        if len(self.types) > 0:
+            types = SubElement(root, "TypeDefinitions")
+            for type_ in self.types:
+                types.append(type_.to_xml())
+
         if len(self.log_categories) > 0:
             categories = SubElement(root, "LogCategories")
             for category, description in self.log_categories.items():
@@ -145,6 +152,8 @@ def __apply_start_value(self, var: ScalarVariable):
             refs = self.get_boolean(vrs)
         elif isinstance(var, String):
             refs = self.get_string(vrs)
+        elif isinstance(var, Enumeration):
+            refs = self.get_integer(vrs)
         else:
             raise Exception(f"Unsupported type!")
 
@@ -172,6 +181,14 @@ def register_variable(self, var: ScalarVariable, nested: bool = True):
         if var.setter is None and hasattr(owner, var.local_name) and var.variability != Fmi2Variability.constant:
             var.setter = lambda v: setattr(owner, var.local_name, v)
 
+    def register_type(self, type: SimpleType):
+        """Register a type as FMU interface.
+
+        Args:
+            type (SimpleType): The type to be registered
+        """
+        self.types.append(type)
+
     def setup_experiment(self, start_time: float, stop_time: Optional[float], tolerance: Optional[float]):
         pass
 
@@ -192,7 +209,7 @@ def get_integer(self, vrs: List[int]) -> List[int]:
         refs = list()
         for vr in vrs:
             var = self.vars[vr]
-            if isinstance(var, Integer):
+            if isinstance(var, Integer) or isinstance(var, Enumeration):
                 refs.append(int(var.getter()))
             else:
                 raise TypeError(
diff --git a/pythonfmu/tests/slaves/pythonslave.py b/pythonfmu/tests/slaves/pythonslave.py
index 8014d0a..8b4379c 100644
--- a/pythonfmu/tests/slaves/pythonslave.py
+++ b/pythonfmu/tests/slaves/pythonslave.py
@@ -1,4 +1,4 @@
-from pythonfmu.fmi2slave import Fmi2Slave, Fmi2Causality, Fmi2Variability, Integer, Real, Boolean, String
+from pythonfmu.fmi2slave import Fmi2Slave, Fmi2Causality, Fmi2Variability, Integer, Real, Boolean, String, Enumeration, TypeEnum, Item
 
 
 class Container:
@@ -19,15 +19,26 @@ def __init__(self, **kwargs):
         self.booleanVariable = True
         self.stringVariable = "Hello World!"
         self.realIn = 2. / 3.
+        self.realIn2 = 2. / 3.
         self.booleanParameter = False
         self.stringParameter = "dog"
+        self.enumParameter = 0
+
+        self.register_type(TypeEnum("enumType", item=[Item("red", 0), Item("green", 1), Item("blue", 2)]))
+
         self.register_variable(
             Integer("intParam", causality=Fmi2Causality.parameter, variability=Fmi2Variability.tunable))
         self.register_variable(Real("realIn", causality=Fmi2Causality.input))
+        self.register_variable(Real("realIn2", causality=Fmi2Causality.input, start=30.0, min_=-100.0, max_=100.0,
+                                    unit="m", display_unit="ft", relative_quantity=True, nominal=10.0, unbounded=False,
+                                    derivative=1, reinit=True))
         self.register_variable(
             Boolean("booleanParameter", causality=Fmi2Causality.parameter, variability=Fmi2Variability.tunable))
         self.register_variable(
             String("stringParameter", causality=Fmi2Causality.parameter, variability=Fmi2Variability.tunable))
+        self.register_variable(
+            Enumeration("enumParameter", causality=Fmi2Causality.parameter, variability=Fmi2Variability.tunable,
+                        declared_type="enumType"))
 
         self.register_variable(Integer("intOut", causality=Fmi2Causality.output))
         self.register_variable(Real("realOut", causality=Fmi2Causality.output))
@@ -47,3 +58,7 @@ def __init__(self, **kwargs):
     def do_step(self, current_time, step_size):
         self.realOut = current_time + step_size
         return True
+
+    def enter_initialization_mode(self):
+        self.intOut = self.intParam
+        self.container.someReal = self.intParam
diff --git a/pythonfmu/type.py b/pythonfmu/type.py
new file mode 100644
index 0000000..4a38a84
--- /dev/null
+++ b/pythonfmu/type.py
@@ -0,0 +1,179 @@
+"""Classes describing interface type."""
+from abc import ABC
+from typing import Any, Optional
+from xml.etree.ElementTree import Element, SubElement
+
+
+class SimpleType(ABC):
+    """Abstract FMI simple type definition.
+
+    Args:
+        name (str): Type name
+        description (str, optional): Type description
+    """
+
+    def __init__(
+            self,
+            name: str,
+            description: Optional[str] = None,
+            getter: Any = None,
+            setter: Any = None
+    ):
+        self.getter = getter
+        self.setter = setter
+        self.local_name = name.split(".")[-1]
+        self.__attrs = {
+            "name": name,
+            "valueReference": None,
+            "description": description,  # Only for ME
+        }
+
+    @property
+    def name(self) -> str:
+        """str: Type name"""
+        return self.__attrs["name"]
+
+    @property
+    def description(self) -> Optional[str]:
+        """str or None: Type description - None if not set"""
+        return self.__attrs["description"]
+
+    def to_xml(self) -> Element:
+        """Convert the type to XML node.
+
+        Returns
+            xml.etree.ElementTree.Element: XML node
+        """
+        attrib = dict()
+        for key, value in self.__attrs.items():
+            if value is not None:
+                attrib[key] = str(value)
+        return Element("SimpleType", attrib)
+
+    def __repr__(self) -> str:
+        return f"{self.__class__.__name__}(name={self.name})"
+
+
+class Real(SimpleType):
+    def __init__(self, name: str, quantity: Optional[str] = None, unit: Optional[str] = None,
+                 display_unit: Optional[str] = None, relative_quantity: Optional[bool] = None,
+                 min_: Optional[float] = None, max_: Optional[float] = None, nominal: Optional[float] = None,
+                 unbounded: Optional[bool] = None, **kwargs):
+        super().__init__(name, **kwargs)
+        self.__attrs = {"quantity": quantity, "unit": unit, "displayUnit": display_unit,
+                        "relativeQuantity": relative_quantity, "min": min_, "max": max_, "nominal": nominal,
+                        "unbounded": unbounded}
+
+    @property
+    def quantity(self) -> Optional[str]:
+        return self.__attrs["quantity"]
+
+    @property
+    def unit(self) -> Optional[str]:
+        return self.__attrs["unit"]
+
+    @property
+    def display_unit(self) -> Optional[str]:
+        return self.__attrs["displayUnit"]
+
+    @property
+    def relative_quantity(self) -> Optional[bool]:
+        return self.__attrs["relativeQuantity"]
+
+    @property
+    def min(self) -> Optional[float]:
+        return self.__attrs["min"]
+
+    @property
+    def max(self) -> Optional[float]:
+        return self.__attrs["max"]
+
+    @property
+    def nominal(self) -> Optional[float]:
+        return self.__attrs["nominal"]
+
+    @property
+    def unbounded(self) -> Optional[bool]:
+        return self.__attrs["unbounded"]
+
+    def to_xml(self) -> Element:
+        attrib = dict()
+        for key, value in self.__attrs.items():
+            if value is not None:
+                # In order to not loose precision, a number of this type should be
+                # stored on an XML file with at least 16 significant digits
+                if key in ["min", "max", "nominal"]:
+                    attrib[key] = f"{value:.16g}"
+                elif key in ["relativeQuantity", "unbounded"]:
+                    attrib[key] = str(value).lower()
+                else:
+                    attrib[key] = str(value)
+        parent = super().to_xml()
+        SubElement(parent, "Real", attrib)
+
+        return parent
+
+
+class Integer(SimpleType):
+    def __init__(self, name: str, quantity: Optional[str] = None, min_: Optional[int] = None,
+                 max_: Optional[int] = None, **kwargs):
+        super().__init__(name, **kwargs)
+        self.__attrs = {"quantity": quantity, "min": min_, "max": max_}
+
+    @property
+    def quantity(self) -> Optional[str]:
+        return self.__attrs["quantity"]
+
+    @property
+    def min(self) -> Optional[int]:
+        return self.__attrs["min"]
+
+    @property
+    def max(self) -> Optional[int]:
+        return self.__attrs["max"]
+
+    def to_xml(self) -> Element:
+        attrib = dict()
+        for key, value in self.__attrs.items():
+            if value is not None:
+                attrib[key] = str(value)
+        parent = super().to_xml()
+        SubElement(parent, "Integer", attrib)
+
+        return parent
+
+
+class Item:
+    def __init__(self, name: str, value: int, description: Optional[str] = None):
+        self.name = name
+        self.value = value
+        self.description = description
+
+
+class Enumeration(SimpleType):
+    def __init__(self, name: str, item: list[Item], quantity: Optional[str] = None, **kwargs):
+        super().__init__(name, **kwargs)
+        self.__attrs = {"quantity": quantity, "Item": item}
+
+    @property
+    def quantity(self) -> Optional[str]:
+        return self.__attrs["quantity"]
+
+    @property
+    def item(self) -> list[Item]:
+        return self.__attrs["Item"]
+
+    def to_xml(self) -> Element:
+        attrib = dict()
+        if self.quantity is not None:
+            attrib["quantity"] = self.quantity
+        parent = super().to_xml()
+        sub = SubElement(parent, "Enumeration", attrib)
+
+        for item in self.item:
+            item_attrib = {"name": item.name, "value": str(item.value)}
+            if item.description is not None:
+                item_attrib["description"] = item.description
+            SubElement(sub, "Item", item_attrib)
+
+        return parent
diff --git a/pythonfmu/variables.py b/pythonfmu/variables.py
index 086a2d9..97e9938 100644
--- a/pythonfmu/variables.py
+++ b/pythonfmu/variables.py
@@ -111,14 +111,64 @@ def __repr__(self) -> str:
 
 
 class Real(ScalarVariable):
-    def __init__(self, name: str, start: Optional[Any] = None, **kwargs):
+    def __init__(self, name: str, start: Optional[float] = None, declared_type: Optional[str] = None,
+                 quantity: Optional[str] = None, unit: Optional[str] = None, display_unit: Optional[str] = None,
+                 relative_quantity: Optional[bool] = None, min_: Optional[float] = None, max_: Optional[float] = None,
+                 nominal: Optional[float] = None, unbounded: Optional[bool] = None, derivative: Optional[int] = None,
+                 reinit: Optional[bool] = None, **kwargs):
         super().__init__(name, **kwargs)
-        self.__attrs = {"start": start}
+        self.__attrs = {"start": start, "declaredType": declared_type, "quantity": quantity, "unit": unit,
+                        "displayUnit": display_unit, "relativeQuantity": relative_quantity, "min": min_, "max": max_,
+                        "nominal": nominal, "unbounded": unbounded, "derivative": derivative, "reinit": reinit}
 
     @property
-    def start(self) -> Optional[Any]:
+    def start(self) -> Optional[float]:
         return self.__attrs["start"]
 
+    @property
+    def declared_type(self) -> Optional[str]:
+        return self.__attrs["declaredType"]
+
+    @property
+    def quantity(self) -> Optional[str]:
+        return self.__attrs["quantity"]
+
+    @property
+    def unit(self) -> Optional[str]:
+        return self.__attrs["unit"]
+
+    @property
+    def display_unit(self) -> Optional[str]:
+        return self.__attrs["displayUnit"]
+
+    @property
+    def relative_quantity(self) -> Optional[bool]:
+        return self.__attrs["relativeQuantity"]
+
+    @property
+    def min(self) -> Optional[float]:
+        return self.__attrs["min"]
+
+    @property
+    def max(self) -> Optional[float]:
+        return self.__attrs["max"]
+
+    @property
+    def nominal(self) -> Optional[float]:
+        return self.__attrs["nominal"]
+
+    @property
+    def unbounded(self) -> Optional[bool]:
+        return self.__attrs["unbounded"]
+
+    @property
+    def derivative(self) -> Optional[int]:
+        return self.__attrs["derivative"]
+
+    @property
+    def reinit(self) -> Optional[bool]:
+        return self.__attrs["reinit"]
+
     @start.setter
     def start(self, value: float):
         self.__attrs["start"] = value
@@ -129,7 +179,12 @@ def to_xml(self) -> Element:
             if value is not None:
                 # In order to not loose precision, a number of this type should be 
                 # stored on an XML file with at least 16 significant digits
-                attrib[key] = f"{value:.16g}"
+                if key in ["start", "min", "max", "nominal"]:
+                    attrib[key] = f"{value:.16g}"
+                elif key in ["relativeQuantity", "unbounded", "reinit"]:
+                    attrib[key] = str(value).lower()
+                else:
+                    attrib[key] = str(value)
         parent = super().to_xml()
         SubElement(parent, "Real", attrib)
 
@@ -137,16 +192,33 @@ def to_xml(self) -> Element:
 
 
 class Integer(ScalarVariable):
-    def __init__(self, name: str, start: Optional[Any] = None, **kwargs):
+    def __init__(self, name: str, start: Optional[int] = None, declared_type: Optional[str] = None,
+                 quantity: Optional[str] = None, min_: Optional[int] = None, max_: Optional[int] = None, **kwargs):
         super().__init__(name, **kwargs)
-        self.__attrs = {"start": start}
+        self.__attrs = {"start": start, "declaredType": declared_type, "quantity": quantity, "min": min_, "max": max_}
 
     @property
-    def start(self) -> Optional[Any]:
+    def start(self) -> Optional[int]:
         return self.__attrs["start"]
 
+    @property
+    def declared_type(self) -> Optional[str]:
+        return self.__attrs["declaredType"]
+
+    @property
+    def quantity(self) -> Optional[str]:
+        return self.__attrs["quantity"]
+
+    @property
+    def min(self) -> Optional[Any]:
+        return self.__attrs["min"]
+
+    @property
+    def max(self) -> Optional[Any]:
+        return self.__attrs["max"]
+
     @start.setter
-    def start(self, value: float):
+    def start(self, value: int):
         self.__attrs["start"] = value
 
     def to_xml(self) -> Element:
@@ -161,23 +233,30 @@ def to_xml(self) -> Element:
 
 
 class Boolean(ScalarVariable):
-    def __init__(self, name: str, start: Optional[Any] = None, **kwargs):
+    def __init__(self, name: str, start: Optional[bool] = None, declared_type: Optional[str] = None, **kwargs):
         super().__init__(name, **kwargs)
-        self.__attrs = {"start": start}
+        self.__attrs = {"start": start, "declaredType": declared_type}
 
     @property
-    def start(self) -> Optional[Any]:
+    def start(self) -> Optional[bool]:
         return self.__attrs["start"]
 
+    @property
+    def declared_type(self) -> Optional[str]:
+        return self.__attrs["declaredType"]
+
     @start.setter
-    def start(self, value: float):
+    def start(self, value: bool):
         self.__attrs["start"] = value
 
     def to_xml(self) -> Element:
         attrib = dict()
         for key, value in self.__attrs.items():
             if value is not None:
-                attrib[key] = str(value).lower()
+                if key == "start":
+                    attrib[key] = str(value).lower()
+                else:
+                    attrib[key] = str(value)
         parent = super().to_xml()
         SubElement(parent, "Boolean", attrib)
 
@@ -185,16 +264,20 @@ def to_xml(self) -> Element:
 
 
 class String(ScalarVariable):
-    def __init__(self, name: str, start: Optional[Any] = None, **kwargs):
+    def __init__(self, name: str, start: Optional[str] = None, declared_type: Optional[str] = None, **kwargs):
         super().__init__(name, **kwargs)
-        self.__attrs = {"start": start}
+        self.__attrs = {"start": start, "declaredType": declared_type}
 
     @property
-    def start(self) -> Optional[Any]:
+    def start(self) -> Optional[str]:
         return self.__attrs["start"]
 
+    @property
+    def declared_type(self) -> Optional[str]:
+        return self.__attrs["declaredType"]
+
     @start.setter
-    def start(self, value: float):
+    def start(self, value: str):
         self.__attrs["start"] = value
 
     def to_xml(self) -> Element:
@@ -206,3 +289,44 @@ def to_xml(self) -> Element:
         SubElement(parent, "String", attrib)
 
         return parent
+
+
+class Enumeration(ScalarVariable):
+    def __init__(self, name: str, start: Optional[int] = None, declared_type: Optional[str] = None,
+                 quantity: Optional[str] = None, min_: Optional[int] = None, max_: Optional[int] = None, **kwargs):
+        super().__init__(name, **kwargs)
+        self.__attrs = {"start": start, "declaredType": declared_type, "quantity": quantity, "min": min_, "max": max_}
+
+    @property
+    def start(self) -> Optional[int]:
+        return self.__attrs["start"]
+
+    @property
+    def declared_type(self) -> Optional[str]:
+        return self.__attrs["declaredType"]
+
+    @property
+    def quantity(self) -> Optional[str]:
+        return self.__attrs["quantity"]
+
+    @property
+    def min(self) -> Optional[int]:
+        return self.__attrs["min"]
+
+    @property
+    def max(self) -> Optional[int]:
+        return self.__attrs["max"]
+
+    @start.setter
+    def start(self, value: int):
+        self.__attrs["start"] = value
+
+    def to_xml(self) -> Element:
+        attrib = dict()
+        for key, value in self.__attrs.items():
+            if value is not None:
+                attrib[key] = str(value)
+        parent = super().to_xml()
+        SubElement(parent, "Enumeration", attrib)
+
+        return parent