Ultrafiltration failure (UFF) is the insufficient ability to remove excess fluid from the body by dialysis. In peritoneal dialysis (PD) this is a common complication, the frequency increases with duration of treatment. In this review a summary of peritoneal transport mechanisms and the conditions associated with UFF are discussed. The 2 most common circumstances in which ultrafiltration failure occurs, peritonitis and long-term PD treatment, are outlined more extensively. In addition a diagnostic approach and therapeutic options for the different causes of UFF are given.