Enter TLA+. TLA+ is a specification language that describes your system and the properties you want. This makes it a fantastic complement to testing: not only can you check your code, you can check your design, too! TLA+ is especially effective for testing concurrency problems, like crashes, race conditions, and dropped messages. This talk will introduce the ideas behind TLA+ and how it works, with a focus on practical examples. We'll also show how it caught complex bugs in our systems, as well as how you can start applying it to your own work.