Asynchronous programming is in demand today, because responsiveness is increasingly important on all modern devices. Yet, we know little about how developers use asynchronous programming in practice. Without such knowledge, developers, researchers, language and library designers, and tool providers can make wrong assumptions. We present the first study that analyzes the usage of asynchronous programming in a large experiment. We analyzed 1378 open source Windows Phone (WP) apps, comprising 12M SLOC, produced by 3376 developers. Using this data, we answer 2 research questions about use and misuse of asynchronous constructs. Inspired by these findings, we developed (i) Asyncifier, an automated refactoring tool that converts callback-based asynchronous code to use async/await; (ii) Corrector, a tool that finds and corrects common misuses of async/await. Our empirical evaluation shows that these tools are (i) applicable and (ii) efficient. Developers accepted 314 patches generated by our tools.@en