RESTful Web services are software services which are published on the Web, taking full advantage and making correct use of the HTTP protocol. This chapter gives an introduction to the REST architectural style and how it can be used to design Web service APIs. We summarize the main design constraints of the REST architectural style and discuss how they impact the design of so-called RESTful Web service APIs. We give examples on how the Web can be seen as a novel kind of software connector, which enables the coordination of distributed, stateful and autonomous software services. We conclude the chapter with a critical overview of a set of emerging technologies which can be used to support the development and operation of RESTful Web services.