Any application that provides users with some form of data that updates over time can implement background fetch. An application that implements background fetch is woken up by iOS periodically, and it's given a small window of time to fetch and process new data that has become available. The OS expects applications to call a callback when they're done with fetching and processing the data. The application uses this callback to inform the OS about whether or not it retrieved new data. First, you will learn about background fetch at a high level. Then, you'll learn about some of the details involved with background fetch, and lastly, you will implement background fetch in the MustC app.