diff --git a/fs.appendFile.spec.js b/fs.appendFile.spec.js
new file mode 100644
index 0000000..5babb9e
--- /dev/null
+++ b/fs.appendFile.spec.js
@@ -0,0 +1,224 @@
+'use strict';
+
+const util = require('../lib/test-utils.js');
+const expect = require('chai').expect;
+
+describe('fs.appendFile', function() {
+  const contents = 'This is a file.';
+
+  beforeEach(function(done) {
+    util.setup(function() {
+      let fs = util.fs();
+      fs.writeFile('/myfile', contents, function(error) {
+        if(error) throw error;
+        done();
+      });
+    });
+  });
+  afterEach(util.cleanup);
+
+  it('should be a function', function() {
+    let fs = util.fs();
+    expect(fs.appendFile).to.be.a('function');
+  });
+
+  it('should append a utf8 file without specifying utf8 in appendFile', function(done) {
+    let fs = util.fs();
+    const more = ' Appended.';
+
+    fs.appendFile('/myfile', more, function(error) {
+      if(error) throw error;
+
+      fs.readFile('/myfile', 'utf8', function(error, data) {
+        expect(error).not.to.exist;
+        expect(data).to.equal(contents + more);
+        done();
+      });
+    });
+  });
+
+  it('should append a utf8 file with "utf8" option to appendFile', function(done) {
+    let fs = util.fs();
+    const more = ' Appended.';
+
+    fs.appendFile('/myfile', more, 'utf8', function(error) {
+      if(error) throw error;
+
+      fs.readFile('/myfile', 'utf8', function(error, data) {
+        expect(error).not.to.exist;
+        expect(data).to.equal(contents + more);
+        done();
+      });
+    });
+  });
+
+  it('should append a utf8 file with {encoding: "utf8"} option to appendFile', function(done) {
+    let fs = util.fs();
+    const more = ' Appended.';
+
+    fs.appendFile('/myfile', more, { encoding: 'utf8' }, function(error) {
+      if(error) throw error;
+
+      fs.readFile('/myfile', { encoding: 'utf8' }, function(error, data) {
+        expect(error).not.to.exist;
+        expect(data).to.equal(contents + more);
+        done();
+      });
+    });
+  });
+
+  it('should append a binary file', function(done) {
+    let fs = util.fs();
+
+    // String and utf8 binary encoded versions of the same thing: 'This is a file.'
+    const binary = Buffer.from([84, 104, 105, 115, 32, 105, 115, 32, 97, 32, 102, 105, 108, 101, 46]);
+    const binary2 = Buffer.from([32, 65, 112, 112, 101, 110, 100, 101, 100, 46]);
+    const binary3 = Buffer.from([84, 104, 105, 115, 32, 105, 115, 32, 97, 32, 102, 105, 108, 101, 46,
+      32, 65, 112, 112, 101, 110, 100, 101, 100, 46]);
+
+    fs.writeFile('/mybinaryfile', binary, function(error) {
+      if(error) throw error;
+
+      fs.appendFile('/mybinaryfile', binary2, function(error) {
+        if(error) throw error;
+
+        fs.readFile('/mybinaryfile', 'ascii', function(error, data) {
+          expect(error).not.to.exist;
+          expect(data).to.deep.equal(binary3);
+          done();
+        });
+      });
+    });
+  });
+
+  it('should follow symbolic links', function(done) {
+    let fs = util.fs();
+    const contents = 'This is a file.';
+    const more = ' Appended.';
+
+    fs.symlink('/myfile', '/myFileLink', function (error) {
+      if (error) throw error;
+
+      fs.appendFile('/myFileLink', more, 'utf8', function (error) {
+        if (error) throw error;
+
+        fs.readFile('/myFileLink', 'utf8', function(error, data) {
+          expect(error).not.to.exist;
+          expect(data).to.equal(contents + more);
+          done();
+        });
+      });
+    });
+  });
+
+  it('should work when file does not exist, and create the file', function(done) {
+    let fs = util.fs();
+
+    fs.appendFile('/newfile', contents, { encoding: 'utf8' }, function(error) {
+      expect(error).not.to.exist;
+
+      fs.readFile('/newfile', 'utf8', function(err, data) {
+        if(err) throw err;
+        expect(data).to.equal(contents);
+        done();
+      });
+    });
+  });
+  
+  it('should accept numbers and append them to the file', function(done) {
+    let fs = util.fs();
+    const more = 10000;
+
+    fs.appendFile('/myfile', more, 'utf8', function(error) {
+      if(error) throw error;
+
+      fs.readFile('/myfile', 'utf8', function(error, data) {
+        expect(error).not.to.exist;
+        expect(data).to.equal(contents + more);
+        done();
+      });
+    });
+  });
+});
+
+describe('fs.promises.appendFile', function() {
+  beforeEach(function(done) {
+    util.setup(function() {
+      let fs = util.fs();
+      return fs.promises.writeFile('/myfile', 'This is a file.', { encoding: 'utf8' })
+        .then(done)
+        .catch(done);
+    });
+  });
+  afterEach(util.cleanup);
+
+  it('should be a function', function() {
+    let fs = util.fs();
+    expect(fs.promises.appendFile).to.be.a('function');
+  });
+
+  it('should append a utf8 file without specifying utf8 in appendFile', function() {
+    let fs = util.fs();
+    const contents = 'This is a file.';
+    const more = ' Appended.';
+
+    return fs.promises.appendFile('/myfile', more)
+      .then(() => fs.promises.readFile('/myfile', 'utf8'))
+      .then(data => expect(data).to.equal(contents + more));
+  });
+
+  it('should append a utf8 file with "utf8" option to appendFile', function() {
+    let fs = util.fs();
+    const contents = 'This is a file.';
+    const more = ' Appended.';
+
+    return fs.promises.appendFile('/myfile', more, 'utf8')
+      .then(() => fs.promises.readFile('/myfile', 'utf8'))
+      .then(data => expect(data).to.equal(contents + more));
+  });
+
+  it('should append a utf8 file with {encoding: "utf8"} option to appendFile', function() {
+    let fs = util.fs();
+    const contents = 'This is a file.';
+    const more = ' Appended.';
+
+    return fs.promises.appendFile('/myfile', more, { encoding: 'utf8' })
+      .then(() => fs.promises.readFile('/myfile', { encoding: 'utf8' }))
+      .then(data => expect(data).to.equal(contents + more));
+  });
+
+  it('should append a binary file', function() {
+    let fs = util.fs();
+
+    // String and utf8 binary encoded versions of the same thing: 'This is a file.'
+    const binary = new Buffer([84, 104, 105, 115, 32, 105, 115, 32, 97, 32, 102, 105, 108, 101, 46]);
+    const binary2 = new Buffer([32, 65, 112, 112, 101, 110, 100, 101, 100, 46]);
+    const binary3 = new Buffer([84, 104, 105, 115, 32, 105, 115, 32, 97, 32, 102, 105, 108, 101, 46,
+      32, 65, 112, 112, 101, 110, 100, 101, 100, 46]);
+
+    return fs.promises.writeFile('/mybinaryfile', binary)
+      .then(() => fs.promises.appendFile('/mybinaryfile', binary2))
+      .then(() => fs.promises.readFile('/mybinaryfile', 'ascii'))
+      .then(data => expect(data).to.deep.equal(binary3));
+  });
+
+  it('should follow symbolic links', function() {
+    let fs = util.fs();
+    const contents = 'This is a file.';
+    const more = ' Appended.';
+
+    return fs.promises.symlink('/myfile', '/myFileLink')
+      .then(() => fs.promises.appendFile('/myFileLink', more, 'utf8'))
+      .then(() => fs.promises.readFile('/myFileLink', 'utf8'))
+      .then(data => expect(data).to.equal(contents + more));
+  });
+
+  it('should work when file does not exist, and create the file', function() {
+    let fs = util.fs();
+    const contents = 'This is a file.';
+
+    return fs.promises.appendFile('/newfile', contents, { encoding: 'utf8' })
+      .then(() => fs.promises.readFile('/newfile', 'utf8'))
+      .then(data => expect(data).to.equal(contents));
+  });
+});