The breast consists of skin, subcutaneous tissue, and glandular tissue. Mammographic density decreases with age as glandular tissue is replaced by fat. Dense breasts have a higher cancer risk and cancers can be hidden. Blood supply comes from internal mammary and axillary arteries, draining to veins and lymphatics. Ultrasound evaluates masses and differentiates cysts from solid lesions. Cysts are anechoic while solid lesions require further evaluation. Benign lesions are circumscribed while malignant lesions demonstrate spiculation, angular margins, and other suspicious signs.