By definition, endoscopic ultrasonography (EUS) combines endoscopy and high-frequency ultrasound, incorporating a small ultrasonic transducer into the tip of endoscopes. For the upper gastrointestinal tract, mostly oblique-viewing endoscopes are used, although recently, forward viewing instruments have become available. For colorectal EUS, rigid probes for the rectum and a flexible forward-viewing echocolonoscope are available. EUS generates ultrasound either mechanically or electronically, depending on the type of instrument used. The electronic technique potentially allows the incorporation of (color) Doppler ultrasound, which allows for additional processing and postprocessing functions. This generally is considered the EUS technique of the future.