The relation of imaging speed and sensitivity to the nature of image processing and the degree to which parallel processing is employed are considered. The limitations imposed on image resolution and ambiguity by the use of a discretely sampled aperture are also considered. Primary parameters in the design of an imaging system include the aperture available, the number of transducer elements employed, and the geometry of the transmitting and receiving aperture sampling functions. Non-ideal response characteristics of the individual elements in the transducer array also produce limitations on the images obtained.