API testing is a software testing technique that involves testing application programming interfaces (APIs) directly and as part of integration testing to determine if they meet expectations for functionality, reliability, performance, and security. The architecture of web APIs promotes the use of formal or informal test cases, where an API has an endpoint with a specification for inputs that are allowed, and the API returns data or output based on those inputs. When determining what to test with an API, it's helpful to understand different categories of tests such as security testing, functional testing, performance testing, and connectivity testing. API monitoring is the process of observing the functional, availability, and performance metrics of an API, and there are many free APIs available for testing. Automated API testing makes development more enjoyable by finding problems early, but it can be challenging to set up and update schemas, input parameters, and data validation. API testing tools have become a solution to these problems, offering features like creating specifications, mocking, and automated testing, with some examples including RapidAPI Testing, Paw, Katalon Studio, SoapUI, and Postman.