Proteinuria occurs commonly after kidney transplantation. Because there are no specific guidelines for defining and detecting proteinuria in transplant recipients, its prevalence can vary depending on the methods used. Most often, the same cutoffs for defining proteinuria in the nontransplant population are applied. There are several risk factors for proteinuria, including some transplant-specific diagnoses and immunosuppressive medications. Posttransplantation proteinuria is associated with reduced graft survival as well as an increased risk of cardiovascular events and death. Treatments to decrease proteinuria have been based on blocking the renin-angiotensin-aldosterone system with the use of angiotensin-converting enzyme inhibitors or angiotensin receptor blockers. This review describes the measurement, prevalence, etiology, prognostic significance, and management of proteinuria in both adult and pediatric transplant recipients.