API testing is a process that confirms an API is working as expected. There are several types of API tests, and each one plays a distinct role in ensuring that the API's functionality, security, and performance remain reliable. Developers can run API tests manually, or they can automate them with an API testing tool.

Traditionally, API testing has occurred at the end of the development phase, but an increasing number of teams are running tests earlier in the API lifecycle. This approach to API testing, which is known as "shifting left," supports rapid iteration by enabling teams to catch and fix issues as soon as they are introduced.

Here, we'll discuss the role that API testing plays in an API-first world—and clarify the relationship between API testing and API monitoring. We'll also review some of the most common approaches to API testing, as well as some best practices. Finally, we'll discuss how the Postman API Platform enables teams to implement an effective API testing strategy that meets their unique needs.

Today's software landscape is highly competitive, and users are increasingly unwilling to tolerate unreliable applications. An issue at an application's API layer can lead to user-facing errors or latency, which can erode customer trust, lead to churn, and negatively impact the business. This puts enormous pressure on development teams to deliver APIs that are consistently available and highly performant.

Many teams have chosen to tackle this challenge by adopting the API-first development model, in which applications are conceptualized and built as a collection of internal and external services that are delivered through APIs. This strategy treats APIs as crucial infrastructure components, which makes API quality a top priority. API testing plays a central role in the API-first approach, as it enables teams to continuously verify the quality, health, and performance of their endpoints as they work to deliver a seamless digital experience.