Congestive heart failure is the only major cardiovascular disease with an increasing incidence and prevalence in industrialized countries. Despite considerable progress in the clinical management of heart failure during the last 20 years, the prognosis is still worse than in many common types of cancer. The kidney is the main organ affected when cardiac function is compromised. In addition, the kidney significantly contributes to the development of the clinical syndrome of heart failure. Specific hemodynamic and neurohormonal abnormalities define the pathophysiology, clinical presentation, and prognosis of this disorder. In this setting, the kidney plays a dual role: the activation of the renin-angiotensin-aldosterone system and the regulation of sodium and water excretion. The kidney is generally intact in heart failure, but extrarenal stimuli alter its function to a point where mechanisms that are initially homeostatic become maladaptive. In this review article, the mechanisms involved in renal adaptation to heart failure are presented.