diff --git a/DESCRIPTION b/DESCRIPTION
index a9af153..929c13a 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,6 +1,6 @@
 Package: hrmn
 Title: Harmonize Datasets
-Version: 0.0.0.9001
+Version: 0.0.0.9002
 Authors@R:
     person("Jon", "Harmon", , "jonthegeek@gmail.com", role = c("aut", "cre"),
            comment = c(ORCID = "0000-0003-4781-4346"))
@@ -22,4 +22,6 @@ Language: en-US
 Roxygen: list(markdown = TRUE)
 RoxygenNote: 7.3.3
 Imports:
-    S7
+    fastmatch,
+    rlang,
+    stbl
diff --git a/NAMESPACE b/NAMESPACE
index 9ebe023..04627fa 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -1,4 +1,6 @@
 # Generated by roxygen2: do not edit by hand
 
+export(harmonize_fct)
 export(specify_fct)
-if (getRversion() < "4.3.0") importFrom("S7", "@")
+importFrom(fastmatch,"%fin%")
+importFrom(rlang,"%||%")
diff --git a/R/aaa-shared_params.R b/R/aaa-shared_params.R
new file mode 100644
index 0000000..eabf2a2
--- /dev/null
+++ b/R/aaa-shared_params.R
@@ -0,0 +1,9 @@
+#' Parameters used in multiple functions
+#'
+#' Reused parameter definitions are gathered here for easier editing.
+#'
+#' @param levels (`character`) The allowed values of the factor.
+#'
+#' @name .shared_params
+#' @keywords internal
+NULL
diff --git a/R/harmonize_fct.R b/R/harmonize_fct.R
new file mode 100644
index 0000000..9e1f9c7
--- /dev/null
+++ b/R/harmonize_fct.R
@@ -0,0 +1,50 @@
+#' Harmonize a factor
+#'
+#' @param .data (`character` or coercible to `character`) A vector to harmonize
+#'   to the specified factor.
+#' @inheritParams .shared_params
+#' @inheritParams rlang::args_dots_empty
+#' @param .spec (`hrmn_fct_spec`) A harmonization specification from
+#'   [specify_fct()].
+#' @param .lookup (named `character`) A vector of replacement values. The names
+#'   are the values in `.data` and the values are the target values.
+#'
+#' @returns A harmonized [factor()].
+#' @export
+#'
+#' @examples
+#' # Without a spec, harmonize_fct() acts like [base::factor()].
+#' harmonize_fct(c("a", "b", "c"))
+#'
+#' # Basic harmonization, dropping levels not in the spec
+#' spec <- specify_fct(levels = c("a", "b"))
+#' harmonize_fct(c("a", "b", "c"), .spec = spec)
+#'
+#' # Using a lookup table to recode values
+#' spec2 <- specify_fct(levels = c("fruit", "citrus"))
+#' lookup <- c(apple = "fruit", banana = "fruit", orange = "citrus")
+#' harmonize_fct(
+#'   c("apple", "banana", "orange"),
+#'   .spec = spec2,
+#'   .lookup = lookup
+#' )
+harmonize_fct <- function(.data, ..., .spec = NULL, .lookup = NULL) {
+  rlang::check_dots_empty()
+  .data <- stbl::to_chr(.data)
+  .spec <- .spec %||% specify_fct()
+  .data <- .apply_fct_lookup(.data, .lookup = .lookup)
+  return(factor(.data, levels = .spec$levels))
+}
+
+#' Apply a lookup table to a character vector
+#'
+#' @inheritParams harmonize_fct
+#' @returns A character vector with values replaced according to the lookup
+#'   table.
+#' @keywords internal
+.apply_fct_lookup <- function(.data, .lookup = NULL) {
+  .lookup <- stbl::to_chr(.lookup)
+  matches <- .data %fin% names(.lookup)
+  .data[matches] <- .lookup[.data[matches]]
+  return(.data)
+}
diff --git a/R/hrmn-package.R b/R/hrmn-package.R
index a65cf64..860748b 100644
--- a/R/hrmn-package.R
+++ b/R/hrmn-package.R
@@ -2,5 +2,7 @@
 "_PACKAGE"
 
 ## usethis namespace: start
+#' @importFrom fastmatch %fin%
+#' @importFrom rlang %||%
 ## usethis namespace: end
 NULL
diff --git a/R/specify_fct.R b/R/specify_fct.R
index 02562ad..293aa14 100644
--- a/R/specify_fct.R
+++ b/R/specify_fct.R
@@ -1,34 +1,19 @@
-# Developer note: `specify_fct()` is designed to create a data-less
-# "specification" object. It defines the target state (the levels) for a factor
-# but doesn't hold any actual factor data itself. This is why the constructor
-# internally provides `integer()` as the data component to `S7::new_object()`.
-#
-# In the future, we might use the `hrmn_fct` class to represent actual,
-# harmonized factor data. In that scenario, we would likely create a separate
-# `class_hrmn_fct` object and have `specify_fct()` be a wrapper function that
-# calls the constructor with the empty data. For now, since we don't need the
-# full factor-like class, we are directly defining the `hrmn_fct` class in
-# `specify_fct()`
-
-#' Specify a factor harmonization
+#' Factor specification
 #'
-#' Create a `hrmn_fct` object that specifies the desired levels for a factor
-#' variable. This 'specification' object does not contain any data itself, only
-#' the rules for harmonization.
+#' Create an object that specifies the desired levels for a factor variable.
+#' This specification object does not contain any data itself, only the rules
+#' for harmonization.
 #'
-#' @param levels (`character`) The allowed values of the factor.
-#' @returns A factor specification, an S7 object of class `hrmn::hrmn_fct`.
+#' @inheritParams .shared_params
+#'
+#' @returns A `hrmn_fct_spec` object that acts as a specification.
 #' @export
-specify_fct <- S7::new_class(
-  "hrmn_fct",
-  parent = S7::class_factor,
-  properties = list(
-    levels = S7::class_character
-  ),
-  constructor = function(levels = character()) {
-    S7::new_object(
-      integer(),
-      levels = levels
-    )
-  }
-)
+#'
+#' @examples
+#' specify_fct(levels = c("a", "b", "c"))
+specify_fct <- function(levels = character()) {
+  structure(
+    list(levels = stbl::to_chr(levels)),
+    class = c("hrmn_fct_spec", "hrmn_spec", "list")
+  )
+}
diff --git a/R/zzz.R b/R/zzz.R
deleted file mode 100644
index 2b31dbb..0000000
--- a/R/zzz.R
+++ /dev/null
@@ -1,3 +0,0 @@
-# enable usage of <S7_object>@name in package code
-#' @rawNamespace if (getRversion() < "4.3.0") importFrom("S7", "@")
-NULL
diff --git a/man/dot-apply_fct_lookup.Rd b/man/dot-apply_fct_lookup.Rd
new file mode 100644
index 0000000..e2dfb44
--- /dev/null
+++ b/man/dot-apply_fct_lookup.Rd
@@ -0,0 +1,23 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/harmonize_fct.R
+\name{.apply_fct_lookup}
+\alias{.apply_fct_lookup}
+\title{Apply a lookup table to a character vector}
+\usage{
+.apply_fct_lookup(.data, .lookup = NULL)
+}
+\arguments{
+\item{.data}{(\code{character} or coercible to \code{character}) A vector to harmonize
+to the specified factor.}
+
+\item{.lookup}{(named \code{character}) A vector of replacement values. The names
+are the values in \code{.data} and the values are the target values.}
+}
+\value{
+A character vector with values replaced according to the lookup
+table.
+}
+\description{
+Apply a lookup table to a character vector
+}
+\keyword{internal}
diff --git a/man/dot-shared_params.Rd b/man/dot-shared_params.Rd
new file mode 100644
index 0000000..8eb0f4b
--- /dev/null
+++ b/man/dot-shared_params.Rd
@@ -0,0 +1,12 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/aaa-shared_params.R
+\name{.shared_params}
+\alias{.shared_params}
+\title{Parameters used in multiple functions}
+\arguments{
+\item{levels}{(\code{character}) The allowed values of the factor.}
+}
+\description{
+Reused parameter definitions are gathered here for easier editing.
+}
+\keyword{internal}
diff --git a/man/harmonize_fct.Rd b/man/harmonize_fct.Rd
new file mode 100644
index 0000000..bb55d28
--- /dev/null
+++ b/man/harmonize_fct.Rd
@@ -0,0 +1,43 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/harmonize_fct.R
+\name{harmonize_fct}
+\alias{harmonize_fct}
+\title{Harmonize a factor}
+\usage{
+harmonize_fct(.data, ..., .spec = NULL, .lookup = NULL)
+}
+\arguments{
+\item{.data}{(\code{character} or coercible to \code{character}) A vector to harmonize
+to the specified factor.}
+
+\item{...}{These dots are for future extensions and must be empty.}
+
+\item{.spec}{(\code{hrmn_fct_spec}) A harmonization specification from
+\code{\link[=specify_fct]{specify_fct()}}.}
+
+\item{.lookup}{(named \code{character}) A vector of replacement values. The names
+are the values in \code{.data} and the values are the target values.}
+}
+\value{
+A harmonized \code{\link[=factor]{factor()}}.
+}
+\description{
+Harmonize a factor
+}
+\examples{
+# Without a spec, harmonize_fct() acts like [base::factor()].
+harmonize_fct(c("a", "b", "c"))
+
+# Basic harmonization, dropping levels not in the spec
+spec <- specify_fct(levels = c("a", "b"))
+harmonize_fct(c("a", "b", "c"), .spec = spec)
+
+# Using a lookup table to recode values
+spec2 <- specify_fct(levels = c("fruit", "citrus"))
+lookup <- c(apple = "fruit", banana = "fruit", orange = "citrus")
+harmonize_fct(
+  c("apple", "banana", "orange"),
+  .spec = spec2,
+  .lookup = lookup
+)
+}
diff --git a/man/specify_fct.Rd b/man/specify_fct.Rd
index f460874..ff3a34e 100644
--- a/man/specify_fct.Rd
+++ b/man/specify_fct.Rd
@@ -2,7 +2,7 @@
 % Please edit documentation in R/specify_fct.R
 \name{specify_fct}
 \alias{specify_fct}
-\title{Specify a factor harmonization}
+\title{Factor specification}
 \usage{
 specify_fct(levels = character())
 }
@@ -10,10 +10,13 @@ specify_fct(levels = character())
 \item{levels}{(\code{character}) The allowed values of the factor.}
 }
 \value{
-A factor specification, an S7 object of class \code{hrmn::hrmn_fct}.
+A \code{hrmn_fct_spec} object that acts as a specification.
 }
 \description{
-Create a \code{hrmn_fct} object that specifies the desired levels for a factor
-variable. This 'specification' object does not contain any data itself, only
-the rules for harmonization.
+Create an object that specifies the desired levels for a factor variable.
+This specification object does not contain any data itself, only the rules
+for harmonization.
+}
+\examples{
+specify_fct(levels = c("a", "b", "c"))
 }
diff --git a/tests/testthat/test-harmonize_fct.R b/tests/testthat/test-harmonize_fct.R
new file mode 100644
index 0000000..a99642c
--- /dev/null
+++ b/tests/testthat/test-harmonize_fct.R
@@ -0,0 +1,90 @@
+test_that("harmonize_fct() works with empty vector and returns a factor", {
+  expect_identical(
+    {
+      harmonize_fct(factor())
+    },
+    factor()
+  )
+})
+
+test_that("harmonize_fct() drops unspecified levels", {
+  expect_identical(
+    {
+      harmonize_fct(factor(c("a", "b")), .spec = specify_fct(levels = "a"))
+    },
+    factor(c("a", NA), levels = "a")
+  )
+})
+
+
+test_that("harmonize_fct() errors if .spec is not named", {
+  expect_error(
+    {
+      harmonize_fct(factor(c("a", "b")), specify_fct(levels = "a"))
+    },
+    class = "rlib_error_dots_nonempty"
+  )
+})
+
+test_that("The first `harmonize_fct()` argument is `.data`", {
+  expect_equal(
+    rlang::fn_fmls_names(harmonize_fct)[1],
+    ".data"
+  )
+})
+
+test_that("harmonize_fct() preserves existing NAs", {
+  expect_equal(
+    {
+      harmonize_fct(factor(c("a", "b", NA)), .spec = specify_fct(levels = "a"))
+    },
+    factor(c("a", NA, NA), levels = "a")
+  )
+})
+
+test_that("harmonize_fct() works with character vectors", {
+  expect_equal(
+    {
+      harmonize_fct(c("a", "b"), .spec = specify_fct(levels = "a"))
+    },
+    factor(c("a", NA), levels = "a")
+  )
+})
+
+test_that("harmonize_fct() works with an empty spec", {
+  expect_equal(
+    {
+      harmonize_fct(
+        factor(c("a", "b")),
+        .spec = specify_fct(levels = character())
+      )
+    },
+    factor(c(NA, NA), levels = character())
+  )
+})
+
+test_that("harmonize_fct() uses .lookup table", {
+  expect_equal(
+    {
+      harmonize_fct(
+        c("x", "y", "z"),
+        .spec = specify_fct(levels = c("a", "b")),
+        .lookup = c(x = "a", y = "a", z = "b")
+      )
+    },
+    factor(c("a", "a", "b"), levels = c("a", "b"))
+  )
+})
+
+test_that("harmonize_fct() .lookup values not in levels become NA", {
+  expect_equal(
+    {
+      harmonize_fct(
+        "x",
+        .spec = specify_fct(levels = "a"),
+        .lookup = c(x = "b")
+      )
+    },
+    factor(NA_character_, levels = "a")
+  )
+})
diff --git a/tests/testthat/test-specify_fct.R b/tests/testthat/test-specify_fct.R
index f966f88..9b93283 100644
--- a/tests/testthat/test-specify_fct.R
+++ b/tests/testthat/test-specify_fct.R
@@ -1,8 +1,7 @@
 test_that("specify_fct() returns an object with the correct class", {
-  spec <- specify_fct()
   expect_s3_class(
-    spec,
-    c("hrmn::hrmn_fct", "factor", "S7_object"),
+    specify_fct(),
+    c("hrmn_fct_spec", "hrmn_spec", "list"),
     exact = TRUE
   )
 })
@@ -10,5 +9,5 @@ test_that("specify_fct() returns an object with the correct class", {
 test_that("specify_fct() stores the levels", {
   lvls <- c("a", "b", "c")
   spec <- specify_fct(levels = lvls)
-  expect_equal(spec@levels, lvls)
+  expect_equal(spec$levels, lvls)
 })