Elastography is a technique that maps relative tissue stiffness. Ultrasonographic (US) elastography (sonoelastography) is a novel modality that is the subject of active research for clinical applications, primarily breast and prostate lesion imaging. Breast and prostate tumors generally have biomechanical properties different from those of normal tissues: Tumors are usually stiffer. This phenomenon is responsible for tissue contrast on elastograms. For the prostate gland and breast, the main image acquisition techniques are vibration sonoelastography and compression sonoelastography. The sonoelastographic appearances of several common breast lesions, including fibroadenomas, simple and complex cysts, ductal carcinomas, malignant lymph nodes, and hematomas, are reviewed. In addition, the US elastographic appearances of the normal prostate gland, prostate carcinomas, and benign prostate hyperplasia are illustrated. Potential pitfalls in the interpretation of elastograms, including false-positive and false-negative images, are illustrated. These imaging findings are derived from ongoing research because sonoelastography is not yet accepted for routine clinical use. © RSNA, 2009.