Publisher Summary Besides rotaviruses (RVs) and enteric adenoviruses (Ad), a number of smaller viruses have increasingly been recognized as important causes of viral gastroenteritis. This chapter discusses the Norwalk-like viruses (NLVs) and Sapporo-like viruses (SLVs) that represent 2 of the 4 genera of the Caliciviridae family. They are the major causes of outbreaks of diarrhea and vomiting in various populations. Human caliciviruses (HuCVs) possess a single-stranded RNA genome of positive polarity and approximately 7,400–7,800 nucleotides in length. There is a major difference between the genomes of NLVs and SLVs in that for SLVs the structural protein open reading frame 2 (ORF2) is in the same reading frame as ORF1 and contiguous with the RNA dependent RNA polymerase (RdRP). For NLVs, ORFs 1 and 2 overlap by a few nucleotides with a −2 frameshift for ORF2. Clinically, HuCV infections start after a short incubation period of 1–2 days with diarrhea and vomiting, projectile vomiting being a characteristic feature. The clinical symptoms are observed in general over a shorter period than that of RV or enteric adenovirus infections. Besides oral or intravenous rehydration therapy there is no specific treatment.