Acute tubulointerstitial nephritis (ATIN) is a common disorder characterized by a spectrum of clinical manifestations ranging from asymptomatic urinary abnormalities to acute oliguric renal failure. Tubular dysfunction out of proportion to the degree of renal failure is an important clue to the diagnosis. This review describes its pathogenesis, pathophysiology, diagnosis, differential diagnosis and therapy, emphasizing the drug-induced form of ATIN.