This article discusses the pathophysiology of sodium and water retention in edematous disorders with a particular focus on cardiac failure, cirrhosis, and pregnancy. The body fluid volume hypothesis, which emphasizes the dominant role of arterial baroreceptors in renal sodium and water excretion, is reviewed. With arterial underfilling, either due to a decrease in cardiac output or peripheral arterial vasodilation, the normal central inhibition of the sympathetic nervous system activity and baroreceptor-mediated, nonosmotic arginine vasopressin (AVP) release is attenuated. The resultant increase in renal adrenergic activity stimulates the renin-angiotensin-aldosterone system. Although the resultant increase in systemic vascular resistance compensates for the primary arterial underfilling, this activation of the neurohumoral axis results in diminished sodium and water delivery to the renal collecting duct sites of aldosterone, AVP, and natriuretic peptide action. This diminished distal sodium and water delivery will be discussed as an important factor in the failure to escape from the sodium-retaining effects of aldosterone, the resistance to the natriuretic and diuretic effects of natriuretic peptides, and the diminished maximal solute-free water excretion in patients with edema. The role of the nonosmotic AVP release in water retention and hypo-osmolality/hyponatremia has been demonstrated in patients and experimental animals by administering nonpeptide, orally active vasopressin V₂ receptor antagonists. These agents have been found to increase solute-free water excretion in patients with water-retaining, hyponatremic edema as well as in experimental animals.