Viral diagnosis is required mainly in the analysis of outbreaks of diarrhea, cases of gastroenteritis in infants and in the exploration of the cause of diarrhea in severely ill patients. Antigens of rotaviruses and adenoviruses can be detected in the feces of the patient, and the rapid tests applied have proven to possess sufficient sensitivity. Sensitivities of the tests intended for norovirus antigen detection have instead remained poor. In addition to antigen detection tests, a real-time PCR test based on the,detection of norovirus nucleic acids has come onto the market, being both easy to use and substantially more sensitive. In the future, multiplex PCR tests allowing simultaneous detection of several different diarrhea-causing microorganisms are expected to become more common.