We have to test more and more functionality in less and less time, as successful software grows with each release, but release cycles are getting shorter.But how can we select the right tests to run at any given moment? Which tests will uncover the most bugs with minimal effort and within the available time?I spent the last decade working on this challenge. Our team has implemented and tried numerous approaches, both with and without AI, in our own development processes and together with customers. In this talk, I will share our failures and successes and what really works.