API testing is one of the most in-demand skills in software testing right now. If you have looked at QA job postings in any sector, you have seen it listed as a requirement or preferred skill. This page explains what it is, why it matters, and how to build verified credentials in it. For a broader look at the field, see our guide on what software testing is.
An API, or Application Programming Interface, is the connection layer that allows different software systems to communicate with each other. When a weather app shows you the forecast, it uses an API to request that data. When you log into a site with your Google account, that authentication uses an API.
The ISTQB glossary defines an API as "a type of interface that defines the way one program may communicate with another." Modern software applications are built on APIs. A single app might interact with dozens. Each connection point is a potential failure point.
API testing is the process of verifying that an API works correctly, returns the right data, handles errors properly, and performs within acceptable parameters. Unlike UI testing, which tests what a user sees and touches, API testing operates at the service layer. The tester sends requests directly to the API and evaluates responses without going through the front end.
API tests run faster than UI tests. They are less likely to break when a visual design changes. They catch defects at the service level before those defects ever surface in the user interface.
A tester sending requests to an API and verifying responses will check:
API testing covers both functional concerns (does the API do what it should?) and non-functional concerns (how fast, how secure, how resilient under load?). Both are covered by the ISTQB Foundation Level syllabus, and non-functional specialist areas including performance testing and security testing have their own ISTQB certifications.
The global software testing and QA services market was valued at approximately $50.6 billion in 2025 and is projected to reach $107.2 billion by 2032. Source: Coherent Market Insights. A significant portion of that growth is driven by API-first development, microservices architecture, and cloud-native applications, all of which multiply the number of API connections in a system, and more connections mean more API testing.
A tester who only knows how to test user interfaces is working with an incomplete picture of how modern software fails. Software testers who build API testing skills are better positioned for the roles the market is growing toward.
Tool knowledge matters. Understanding the principles of what good API testing looks like matters more. The ISTQB Foundation Level syllabus covers the underlying concepts that apply regardless of which tool a team uses. There is no dedicated ISTQB API Testing certification, so the closest ISTQB fit is often Technical Test Analyst or Test Automation Engineer. If your work overlaps with broader automation coverage, read What Is Test Automation?.
AT*SQA offers a series of API testing micro-credentials that each result in a verifiable, shareable credential. They can be taken individually or combined as the AT*SQA API Testing Certification.
Each ISTQB exam purchase through AT*SQA includes a free micro-credential of your choice. API testing is one of the available options. If you are planning to sit the ISTQB Foundation Level exam, you can add an API testing credential at no extra cost. If you want another practical take on why that matters, TestingCred has a useful piece on why testers should earn an API Testing micro-credential. If you are still choosing your path, use Which ISTQB Certification Should I Take?. If your API work leans more technical or performance-heavy, compare Technical Test Analyst and Performance Testing. See everything included with AT*SQA exam purchase.
For context on how API testing fits the broader job market, see How to Become a Software Tester, how ISTQB certification helps on your resume, and whether ISTQB is worth it.