Body cavity effusions occur when there is abnormal accumulation of fluid in a body cavity. In dogs and cats, effusions commonly occur in the pleural, peritoneal or pericardeal spaces. Clinical signs, such as dyspnoea, lethargy, exercise intolerance and abdominal distension, can be due to the presence of the effusion, the disease responsible for producing the effusion or both. The chapter looks at the pathophysiology of effusion formation, collection of body cavity effusions, laboratory evaluation and the classification of effusions. This chapter also includes case examples.