**Application programming interfaces** (APIs) are interfaces between clients and servers or applications and operating systems that define how the client should ask for information from the server and how the server will respond. APIs are tremendously useful for *building interfaces between systems*, but they can also be a point of vulnerability if they are not properly secured. API security relies on *authentication, authorisation, proper data scoping to ensure that too much data isn’t released, rate limiting, input filtering, and appropriate monitoring and logging* to remain secure.