Acute interstitial nephritis (AIN) is a common cause of acute kidney injury. Many etiologies of AIN have been recognized--including allergic/drug-induced, infectious, autoimmune/systemic, and idiopathic forms of disease. The most common etiology of AIN is drug-induced disease, which is thought to underlie 60-70% of cases. Multiple agents from many different classes of drugs can cause AIN, and the clinical presentation and laboratory findings vary according to the class of drug involved. AIN is characterized by interstitial inflammation, tubulitis, edema, and in some cases, eventual interstitial fibrosis. A definitive diagnosis of AIN can be established only by kidney biopsy. Noninvasive tests such as (67)gallium scintigraphy and testing for eosinophiluria have limited diagnostic utility. The mainstay of therapy for drug-induced AIN is timely discontinuation of the causative agent. Although the benefits of corticosteroid therapy remain unproven, they do appear to have a positive effect in some patients with drug-induced AIN, especially when treatment is initiated early in the course of the disease. In general, the prognosis for drug-induced AIN is good, and at least partial recovery of kidney function is normally observed. Early recognition is crucial because patients can ultimately develop chronic kidney disease.