Ultrasonic images are imperfect because practical imaging systems have limited spatial, contrast, and temporal resolutions. The envelope within which an ultrasonic imaging system operates is imposed by the physical and biological properties of the imaged tissue. The relevant properties are speed, attenuation, inhomogeneity, nonlinearity, scattering, motion, and the induction of biological damage. The system designer begins by choosing the overall dimensions of the tissue structures to be imaged and the imaging rate. Then, optimization of system design allows the imaging performance to coincide with the envelope imposed by the characteristics of the tissue.