Medical devices are a very large family of instruments, machines, systems, reagents and much more, all characterized by having a specific medical purpose. This definition is not univocal and there are changes in the different regulatory contexts of individual countries. In this chapter we will mainly refer to the definitions of medical devices according to European legislation, but we will also make some references to other nations, namely the United States and China. We will also describe the different categories of medical devices and some of their characteristics. The classification rules defined by the European MDR and IVDR regulations will be introduced, both for medical devices and for in vitro diagnostics, and numerous examples of devices will be reported with the indication of the corresponding risk class. Finally, we will present the typical development process of a medical device, from conception to placing it on the market.