The rails "official stack" tests with minitest. Each revision of rails peels back the testing onion and encourages better testing practices. Rails 4.0 switched to minitest 4, Rails 4.1 switched to minitest 5, and Rails 4.2 switched to randomizing the test run order. I'll explain what has happened, explain the motivation behind the changes, and how to diagnose and solve problems you may have as you upgrade. Whether you use minitest already, are considering switching, or just use rspec and are curious what's different, this talk will have something for you.