Canine leptospirosis has been known since 1899. Widespread use of canine leptospiral vaccines decreased the incidence of infection dramatically and reduced clinical attention to the disease. However, leptospirosis recently has reemerged as an important cause of febrile illness, and hepatic and renal disease in dogs. Feline leptospirosis is rare although the incidence of infection is higher than is recognized. Diagnosis of leptospirosis is difficult and no single diagnostic test provides optimal sensitivity or specificity. A combination of procedures, including serological assays and tests to detect the presence of leptospires in tissues or body fluids, is recommended.