Acute interstitial nephritis (AIN) represents a frequent cause of acute kidney injury, accounting for 15-27% of renal biopsies performed because of this condition. By and large, drug-induced AIN is currently the commonest etiology of AIN, with antimicrobials and nonsteroidal anti-inflammatory drugs being the most frequent offending agents. Pathogenesis is based on an immunologic reaction against endogenous nephritogenic antigens or exogenous antigens processed by tubular cells, with cell-mediated immunity having a major pathogenic role. The characteristic interstitial infiltrates, mostly composed of lymphocytes, macrophages, eosinophils, and plasma cells, experience a rapid transformation into areas of interstitial fibrosis. A significant proportion of AIN has nowadays an oligosymptomatic presentation, although the presence of specific extrarenal symptoms such as fever, skin rash, arthralgias, and peripheral eosinophilia has an important role to orientate clinical diagnosis. Identification and removal of the offending drug are the mainstay of the treatment, but recent studies strongly suggest that early steroid administration (within 7 days after diagnosis) improves the recovery of renal function, decreasing the risk of chronic renal impairment. Delayed steroid treatment, when interstitial fibrosis has taken place, would have a less pronounced or nule therapeutic benefit.