Patients with altered renal function are frequently encountered in the emergency department (ED) and emergency physicians often play an important role in the evaluation and management of renal disease. Early recognition, diagnosis, prevention of further iatrogenic injury, and management of renal disease have important implications for long-term morbidity and mortality. This article reviews basic renal physiology, discusses the differential diagnosis and approach to therapy, as well as strategies to prevent further renal injury, for adult patients who present to the ED with renal injury or failure.