RADIUS (Remote Authentication Dial In User Service) is a AAA system. It is used to check the identity of a user or device that wants access to the network. For example, on your routers and switches you have usernames and passwords to authenticate the network engineers. Instead of locally configuring all usernames/passwords, you can also use a RADIUS server. When a network engineer tries to log in a router or switch, the credentials will be checked against the RADIUS server which decides if the engineer gets access or not. RADIUS servers are also used commonly used in wireless networking, where users are authenticated.