Pericardial disease is common, with reports in referral institutions of 8% of cardiac cases in dogs and 6% of feline cases. Congenital, acquired primary and acquired secondary forms have been described. The most common pericardial diseases result in accumulation of pericardial fluid. In dogs, primary acquired disease is most common, where local (often inflammatory or neoplastic) processes are responsible. In cats, secondary acquired disease such as congestive heart failure (CHF) predominates. Pericardial disease can often be mistaken for other cardiac or non-cardiac disorders and diagnosis can be challenging. The following subjects are discussed: Normal pericardium; Pathophysiology; Causes; Clinical signs; Diagnostic approach; Pericardiocentesis; and Long-term treatment and prognosis.