The main purpose of signal processing in ultrasound imaging is the improvement of the image quality. The basics of three-dimensional, nonlinear image filtering are described in a simple way. For Doppler several million operations are computed per second to obtain the velocity distribution as a function of time. Various methods are described for overcoming the alias problem. For Doppler imaging the necessary computing power increases to several hundred million operations per second. Like other medical imaging methods, diagnostic ultrasound depends on powerful electronic computers.