Swift is a fairly new language, and the Apple developers keep updating the underlying APIs and language syntax while enhancing the language features by adding new APIs. In this chapter, we learned about the changes and additions to Swift with its latest release, that is, Swift 4. We covered how to migrate our current IDE to support the latest Swift release, and then we went ahead and covered major changes to the existing APIs, followed by the new additions to the language. We highlighted the major changes that you will encounter most in your day-to-day programming practices. Toward the end of the chapter, we also covered how to migrate an existing code base to Swift 4 and tackle some of the issues that you might come across while trying to update the code base. This book is more about Reactive programming, so we will start working with Reactive concepts from Chapter 2, FRP Fundamentals, Terminology, and Basic Building Blocks, onward, but since we will be working with Swift 4 throughout the book, it made sense to start by getting you up to date with the development language; next up will be the setup of the overall development environment.