diff --git a/.Rbuildignore b/.Rbuildignore
index d6bbaf8..d27a7b6 100644
--- a/.Rbuildignore
+++ b/.Rbuildignore
@@ -8,3 +8,4 @@
 ^pkgdown$
 ^\.github$
 ^LICENSE\.md$
+^codecov\.yml$
diff --git a/.github/workflows/test-coverage.yaml b/.github/workflows/test-coverage.yaml
new file mode 100644
index 0000000..4e0ec42
--- /dev/null
+++ b/.github/workflows/test-coverage.yaml
@@ -0,0 +1,51 @@
+# Workflow derived from https://github.com/r-lib/actions/tree/v2/examples
+# Need help debugging build failures? Start at https://github.com/r-lib/actions#where-to-find-help
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+    branches: [main, master]
+
+name: test-coverage
+
+jobs:
+  test-coverage:
+    runs-on: ubuntu-latest
+    env:
+      GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
+      CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: r-lib/actions/setup-r@v2
+        with:
+          use-public-rspm: true
+
+      - uses: r-lib/actions/setup-r-dependencies@v2
+        with:
+          extra-packages: any::covr
+          needs: coverage
+
+      - name: Test coverage
+        run: |
+          covr::codecov(
+            quiet = FALSE,
+            clean = FALSE,
+            install_path = file.path(normalizePath(Sys.getenv("RUNNER_TEMP"), winslash = "/"), "package")
+          )
+        shell: Rscript {0}
+
+      - name: Show testthat output
+        if: always()
+        run: |
+          ## --------------------------------------------------------------------
+          find '${{ runner.temp }}/package' -name 'testthat.Rout*' -exec cat '{}' \; || true
+        shell: bash
+
+      - name: Upload test results
+        if: failure()
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-test-failures
+          path: ${{ runner.temp }}/package
diff --git a/DESCRIPTION b/DESCRIPTION
index a03670a..ddb6b2a 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -26,3 +26,6 @@ Imports:
 URL: https://rmgpanw.github.io/trud/,
     https://github.com/rmgpanw/trud
 BugReports: https://github.com/rmgpanw/trud/issues
+Suggests: 
+    testthat (>= 3.0.0)
+Config/testthat/edition: 3
diff --git a/README.Rmd b/README.Rmd
index 0384c67..96c7b45 100644
--- a/README.Rmd
+++ b/README.Rmd
@@ -16,6 +16,7 @@ knitr::opts_chunk$set(
 # trud <a href="https://rmgpanw.github.io/trud/"><img src="man/figures/logo.png" align="right" height="138"/></a>
 
 <!-- badges: start -->
+[![Codecov test coverage](https://codecov.io/gh/rmgpanw/trud/branch/main/graph/badge.svg)](https://app.codecov.io/gh/rmgpanw/trud?branch=main)
 <!-- badges: end -->
 
 The goal of trud is to provide a convenient R interface to the [NHS TRUD API](https://isd.digital.nhs.uk/trud/users/guest/filters/0/api).
diff --git a/codecov.yml b/codecov.yml
new file mode 100644
index 0000000..04c5585
--- /dev/null
+++ b/codecov.yml
@@ -0,0 +1,14 @@
+comment: false
+
+coverage:
+  status:
+    project:
+      default:
+        target: auto
+        threshold: 1%
+        informational: true
+    patch:
+      default:
+        target: auto
+        threshold: 1%
+        informational: true
diff --git a/tests/testthat.R b/tests/testthat.R
new file mode 100644
index 0000000..4d32dcd
--- /dev/null
+++ b/tests/testthat.R
@@ -0,0 +1,12 @@
+# This file is part of the standard setup for testthat.
+# It is recommended that you do not modify it.
+#
+# Where should you do additional test configuration?
+# Learn more about the roles of various files in:
+# * https://r-pkgs.org/testing-design.html#sec-tests-files-overview
+# * https://testthat.r-lib.org/articles/special-files.html
+
+library(testthat)
+library(trud)
+
+test_check("trud")
diff --git a/tests/testthat/test-utils.R b/tests/testthat/test-utils.R
new file mode 100644
index 0000000..a8238c6
--- /dev/null
+++ b/tests/testthat/test-utils.R
@@ -0,0 +1,4 @@
+test_that("get_trud_api_key() raises error with non-string argument", {
+  expect_error(get_trud_api_key(1),
+               "Argument `TRUD_API_KEY` must either be a string or `NULL`")
+})