Pericardial effusion refers to the presence of fluid in the pericardial space. There are many causes of pericardial effusion including idiopathic (young dogs), neoplasia, infectious, and rarely traumatic. In cats with congestive heart failure, it is not uncommon to occasionally find a small amount of pericardial effusion, and this is the most common cause of pericardial effusion in this species. Cardiac tamponade occurs when there is excess pericardial effusion, which causes significant compression of the heart itself. The effusion in the majority of cases is blood, but occasionally it can be serosanguinous or purulent. Pericardial effusion may be suspected based on the history and physical findings. Thoracic radiographs demonstrate cardiomegaly, and ultrasonography definitively demonstrates fluid in the pericardium. Immediate drainage of the pericardial sac is the only treatment to relieve the symptoms associated with cardiac tamponade in an emergency situation.