The aquatic environment has a long and documented history in rehabilitation. This environment causes significant biologic effects which are applicable to many rehabilitative problems. Both immediate and delayed physiologic effects are noted in the immersed human and involve nearly all the basic homeostatic mechanisms. Particularly affected are the cardiovascular, renal, and pulmonary systems. The fundamental cardiovascular process is an increase in venous return with consequent increases in right atrial pressure, stroke volume, and cardiac output. The net effects upon the renal system are decreased anti-diuretic hormone (ADH) and aldosterone production, causing increased sodium and potassium excretion and consequent diuresis. The effects upon the respiratory system result from hydrostatic compression of the thorax coupled with increased intrathoracic blood volume. Both increase the work of breathing and decrease expiratory reserve volume. Effects upon muscle circulation, joint unloading, and general conditioning are also potentially useful in rehabilitation. This article reviews these effects and the literature support for these biologic changes.