Ascites is the abnormal accumulation of fluid in the peritoneal cavity. It is readily identified on abdominal ultrasound as anechoic free fluid in dependent areas including the right upper quadrant (Morrison’s pouch), the left upper quadrant (splenorenal space), and the pelvis (pouch of Douglas). Ultrasound can detect as little as 100 mL of free fluid. Common causes include liver cirrhosis, malignancy, heart failure, and peritoneal disease. Ultrasound is also used to guide diagnostic or therapeutic paracentesis.