Continuous integration (CI) testing has evolved significantly from its early days of manual code reviews in the 1980s to the automated, cloud-based systems of today. Initially, testing was a slow, manual process focused on finding errors through code inspections. The 1990s saw the rise of unit tests written by specialized testers, but feedback was often delayed due to slow test execution. The cultural shift towards developers writing their own tests was spurred by Extreme Programming in the late 1990s, leading to quicker feedback loops. Automation gained traction in the early 2000s with tools like Jenkins, enabling the first generation of CI tests that automatically verified code changes. By the late 2000s, CI moved to the cloud, allowing smaller teams to benefit from testing services without maintaining their infrastructure. Modern CI practices focus on optimizing test speed through vertical scaling, parallelization, and caching, with cloud providers offering scalable resources. Some high-velocity organizations have begun experimenting with batching and skipping certain tests to enhance speed further. The advent of AI might introduce even faster code review processes by probabilistically predicting outcomes, potentially transforming CI testing once again.