The PID relay auto-tuner of Astrom–Hagglund is one of the simplest and most robust auto-tuning techniques for process controllers and has been successfully applied to industry for more than 15 years. This tuner is based on an approximate estimation of the critical point on the process frequency response from relay oscillations. Many developments have recently been reported to extend its applications. It turns out that more and accurate information on process dynamics can be obtained from the same relay test with the help of new identification techniques, and used to tune PID controllers better. Extensions are also made to tune model-based advanced controllers and multivariable controllers. The present paper reviews these developments and shows the state-of art in relay auto-tuning of process controllers.