What Is Regression Testing? The Definitive Guide (2026) Regression testing is the systematic verification that a change made to software — a bug fix, a new feature, or a dependency update — has not introduced defects in parts of the system that previously worked. You just shipped a feature. The client is happy. The team celebrates. And then, forty-eight hours later, support blows up: the payment form no longer works. Nobody touched it. But the code you added elsewhere broke everything, silently. This scenario is not hypothetical. It's the daily reality of thousands of development teams. And it's exactly what regression testing is supposed to prevent. This guide covers everything you need to know: the definition, the different types, the ideal time to run it, automation strategies — and most importantly, the type of regression that almost everyone ignores even though it's the most visible to your users.…