The athlete's heart is a constellation of cardiac morphologic changes, including increased left ventricular volume, increased left ventricular mass, increased left atrial volume, and right ventricular structural changes as a physiologic response to exercise training. These structural changes fall within the normal reference ranges of appropriately matched control subjects for most trained individuals; however, there are significant numbers of athletes who have "abnormal" measurements. The ability to distinguish between physiologic changes associated with the athlete's heart and structural abnormalities that may represent underlying cardiac disease is of paramount importance. Structural heart disease significantly increases risk for morbidity, mortality, and sudden death, especially during exercise or physical stress. This article reviews the morphologic changes associated with the athlete's heart.