Although most cardiac diseases have an adverse effect on cardiac output, it is congestive heart failure (CHF) that is usually responsible for clinical signs in dogs and cats. CHF can be defined as abnormal fluid accumulation as a result of heart disease. Increased atrial pressures lead to increased capillary hydrostatic pressure, and fluid accumulates in the interstitial spaces (pulmonary oedema) or body cavities (pleural effusion or ascites). Congestive signs are more common than signs of low cardiac output because the main priority of the cardiovascular system is provision of adequate systemic arterial blood pressure, and this is tightly regulated. Arterial blood pressure is maintained in the face of reduced cardiac output via activation of a number of neurohormonal mechanisms. These include increased sympathetic tone, as well as other mechanisms that help to increase cardiac output (such as sodium and water retention) or increase systemic vascular resistance. Importantly, support of arterial blood pressure is often at the expense of increased filling (atrial) pressures, increased risk of pulmonary oedema or effusions, and increased myocardial work. This chapter considers Treatment according to stage of heart disease; and Specific treatments.