The performance of modern ultrasound imaging systems is primarily determined by the characteristics of the ultrasound transducer. Early imaging systems were based on single-element transducers. These transducers had a fixed focal distance and had to be mechanically scanned to create an image. The systems were cumbersome to use, and the image quality was relatively poor. Much of the success of modern ultrasound imaging can be attributed to the development of efficient transducer arrays. The ultrasound beam produced by an array can be electronically steered and focused. Electronic steering avoids the need to mechanically scan the transducer, whereas electronic focusing provides improved resolution over a large imaging depth. Since transducer arrays were first introduced in the late 1970s, there has been a steady improvement in the efficiency and resolution of arrays. Transducer arrays are now available in a variety of geometries and operating frequencies, each optimized for a particular imaging application. As new transducer materials and improved fabrication techniques have become available, there has been a corresponding improvement in the quality of the ultrasound images. The pace of this development has not slowed, and improvements in ultrasound imaging can be expected for many years to come.