APIs are consumed by programs, but those programs are built by humans. A good API is one that both programs and humans enjoy using. That means the API not only has to be powerful and responsive, but usable, too. Using examples like Twilio, Twitter, and FourSquare, we'll discuss what makes an API usable or not. We'll talk about common usability traps and the bugs we make or prevent consuming applications. Finally, we'll discuss approaches to API development that improve usability.