In order to interpret breast sonograms successfully, it is necessary to understand the technical factors that affect the image. It is especially important to be aware of the effects of focal zone, power, time gain compensation, and gray scale selection on cyst/solid differentiation, the primary indication for breast sonography. Adequate breast examinations can be performed with either high-frequency automated whole-breast ultrasound units or hand-held instruments with a transducer operating at 5 MHz or greater frequency, and with a depth of focus of no more than 3 cm. Familiarity with the normal breast sonogram, including the variations in the relative amounts of parenchyma, connective tissue, and fat that occur with increasing age and parity, is essential for the identification, characterization, and localization of abnormalities.