diff --git a/__mocks__/@actions/github.ts b/__mocks__/@actions/github.ts
new file mode 100644
index 0000000..5ff9a86
--- /dev/null
+++ b/__mocks__/@actions/github.ts
@@ -0,0 +1,32 @@
+export const context = {
+  payload: {
+    pull_request: {
+      number: 123,
+    },
+  },
+  repo: {
+    owner: "monalisa",
+    repo: "helloworld",
+  },
+};
+
+const mockApi = {
+  issues: {
+    addLabels: jest.fn(),
+    removeLabel: jest.fn(),
+  },
+  paginate: jest.fn(),
+  pulls: {
+    get: jest.fn().mockResolvedValue({}),
+    listFiles: {
+      endpoint: {
+        merge: jest.fn().mockReturnValue({}),
+      },
+    },
+  },
+  repos: {
+    getContents: jest.fn(),
+  },
+};
+
+export const GitHub = jest.fn().mockImplementation(() => mockApi);
diff --git a/__tests__/fixtures/only_pdfs.yml b/__tests__/fixtures/only_pdfs.yml
new file mode 100644
index 0000000..1bcce76
--- /dev/null
+++ b/__tests__/fixtures/only_pdfs.yml
@@ -0,0 +1,2 @@
+touched-a-pdf-file:
+  - any: ['*.pdf']
diff --git a/__tests__/main.test.ts b/__tests__/main.test.ts
new file mode 100644
index 0000000..f4e895d
--- /dev/null
+++ b/__tests__/main.test.ts
@@ -0,0 +1,116 @@
+import { run } from "../src/labeler";
+import { GitHub } from "@actions/github";
+import * as core from "@actions/core";
+
+const fs = jest.requireActual("fs");
+
+jest.mock("@actions/core");
+jest.mock("@actions/github");
+
+const gh = new GitHub("_");
+const addLabelsMock = jest.spyOn(gh.issues, "addLabels");
+const removeLabelMock = jest.spyOn(gh.issues, "removeLabel");
+const reposMock = jest.spyOn(gh.repos, "getContents");
+const paginateMock = jest.spyOn(gh, "paginate");
+const getPullMock = jest.spyOn(gh.pulls, "get");
+
+const yamlFixtures = {
+  "only_pdfs.yml": fs.readFileSync("__tests__/fixtures/only_pdfs.yml"),
+};
+
+afterAll(() => jest.restoreAllMocks());
+
+describe("run", () => {
+  it("adds labels to PRs that match our glob patterns", async () => {
+    usingLabelerConfigYaml("only_pdfs.yml");
+    mockGitHubResponseChangedFiles("foo.pdf");
+
+    await run();
+
+    expect(removeLabelMock).toHaveBeenCalledTimes(0);
+    expect(addLabelsMock).toHaveBeenCalledTimes(1);
+    expect(addLabelsMock).toHaveBeenCalledWith({
+      owner: "monalisa",
+      repo: "helloworld",
+      issue_number: 123,
+      labels: ["touched-a-pdf-file"],
+    });
+  });
+
+  it("does not add labels to PRs that do not match our glob patterns", async () => {
+    usingLabelerConfigYaml("only_pdfs.yml");
+    mockGitHubResponseChangedFiles("foo.txt");
+
+    await run();
+
+    expect(removeLabelMock).toHaveBeenCalledTimes(0);
+    expect(addLabelsMock).toHaveBeenCalledTimes(0);
+  });
+
+  it("(with sync-labels: true) it deletes preexisting PR labels that no longer match the glob pattern", async () => {
+    let mockInput = {
+      "repo-token": "foo",
+      "configuration-path": "bar",
+      "sync-labels": true,
+    };
+
+    jest
+      .spyOn(core, "getInput")
+      .mockImplementation((name: string, ...opts) => mockInput[name]);
+
+    usingLabelerConfigYaml("only_pdfs.yml");
+    mockGitHubResponseChangedFiles("foo.txt");
+    getPullMock.mockResolvedValue(<any>{
+      data: {
+        labels: [{ name: "touched-a-pdf-file" }],
+      },
+    });
+
+    await run();
+
+    expect(addLabelsMock).toHaveBeenCalledTimes(0);
+    expect(removeLabelMock).toHaveBeenCalledTimes(1);
+    expect(removeLabelMock).toHaveBeenCalledWith({
+      owner: "monalisa",
+      repo: "helloworld",
+      issue_number: 123,
+      name: "touched-a-pdf-file",
+    });
+  });
+
+  it("(with sync-labels: false) it issues no delete calls even when there are preexisting PR labels that no longer match the glob pattern", async () => {
+    let mockInput = {
+      "repo-token": "foo",
+      "configuration-path": "bar",
+      "sync-labels": false,
+    };
+
+    jest
+      .spyOn(core, "getInput")
+      .mockImplementation((name: string, ...opts) => mockInput[name]);
+
+    usingLabelerConfigYaml("only_pdfs.yml");
+    mockGitHubResponseChangedFiles("foo.txt");
+    getPullMock.mockResolvedValue(<any>{
+      data: {
+        labels: [{ name: "touched-a-pdf-file" }],
+      },
+    });
+
+    await run();
+
+    expect(addLabelsMock).toHaveBeenCalledTimes(0);
+    expect(removeLabelMock).toHaveBeenCalledTimes(0);
+  });
+});
+
+function usingLabelerConfigYaml(fixtureName: keyof typeof yamlFixtures): void {
+  reposMock.mockResolvedValue(<any>{
+    data: { content: yamlFixtures[fixtureName], encoding: "utf8" },
+  });
+}
+
+function mockGitHubResponseChangedFiles(...files: string[]): void {
+  const returnValue = files.map((f) => ({ filename: f }));
+  paginateMock.mockReturnValue(<any>returnValue);
+}
diff --git a/tsconfig.json b/tsconfig.json
index 9cde892..0018cde 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -59,5 +59,5 @@
     // "experimentalDecorators": true,        /* Enables experimental support for ES7 decorators. */
     // "emitDecoratorMetadata": true,         /* Enables experimental support for emitting type metadata for decorators. */
   },
-  "exclude": ["node_modules", "**/*.test.ts"]
+  "exclude": ["node_modules", "__tests__", "__mocks__"]
 }