Ascites is a clinical symptom, a major complication of many medical conditions in dogs and cats. It is characterized by an enlarged abdominal outline due to the accumulation of an excessive amount of fluid in the abdominal cavity. The fluid may be an effusion, exudate, blood, lymph or urine. This clinical review describes the pathophysiological mechanisms underlying the formation of excessive amounts of fluid and indicates the most common causes of heart, liver, kidney diseases and neoplasms in small animals. The article presents the diagnostic tests aimed at identifying the causative agent, which include a physical examination, blood chemistry studies, urinalysis, ultrasound studies and examination of a sample of the abdominal fluid. The methods of symptomatic treatment are also described independently of the necessary treatment addressing the cause of ascites.