The main objective of this thesis was to investigate the etiology of outbreaks of viral gastroenteritis that remained without diagnosis after testing for common viral pathogens causing gastroenteritis, e.g. noroviruses, rotaviruses, sapoviruses, adenoviruses, and astroviruses. No causative pathogen could be detected in over 10% of these outbreaks. Viral gastroenteritis is about the most frequent disease in humans and this unexplained fraction concerns about 150000 cases in the Netherlands each year. Here, an overview of the state of the art of the literature on viral gastroenteritis is provided.