The basics of ultrasonic transducer array design in the frequency range useful for medical imaging are discussed. Performance parameters of importance in transducer design are considered, including sensitivity, coupling constant, band width, frequency downshift, pulse duration, beam focusing properties, and electrical matching. 2D and 3D effects must also be taken into account. The advantages of computer modeling in 3D with finite element analysis code are highlighted. The principles of multi-element array transducers useful for 2D real-time scanning are reviewed. 2D arrays provide the opportunity of focusing the elevation beam dimension in the short axis, or make possible full 3D scanning volumes.