Lyssavirus infection, better known as rabies, in animals or in humans, is characterized by a clinical picture of viral encephalitis. According to the WHO estimations, it is responsible for more than 50,000 human deaths each year. The diagnosis of lyssavirus infection in animals, which may be responsible for human infections are of utmost importance: if positive, for starting or follow-up of postexposure rabies treatments, or if negative, for the nonprescription or interruption of the treatments. The diagnosis of lyssavirus infection in humans, despite the lack of antivirus treatment after the onset of the disease, eliminates the expense and discomfort of unnecessary investigative tests and inappropriate therapy, and encourages the early initiation of specific epidemiologic control measures such as maintenance of strict isolation precautions and education of employees. This helps to reduce significantly the number of postexposure prophylaxis in hospital personnel. However, the signs and symptoms of the rabies encephalitis may vary greatly according to species and individuals. The clinical differential diagnosis with other viral encephalitis may often be difficult if not impossible. Therefore, laboratory methods are necessary to confirm lyssavirus infection. Nevertheless, some requirements are necessary to ascertain the reliability of the laboratory results: good quality of specimens, good transport conditions to the laboratory, the short delay necessary to obtain the results, and the quick communication of the results.