Picobirnaviruses (PBV) are a group of small, nonenveloped viruses with bi-segmented dsRNA genomes. The large genomic segment encodes for the capsid protein while the small segment encodes for the RNA-dependent RNA polymerase. PBV are widely distributed and have been detected in the feces of a large variety of vertebrate species, including farm and captive animals and also in humans. Their etiological role as a cause of diarrhea remains elusive. Still, compelling evidence indicates that PBV may be opportunistic pathogens associated with diarrhea in immunocompromised individuals. The partial molecular characterization of human and animal strains revealed that PBVs are highly variable, and at least two distinct genogroups have been recognized. In addition, the possibility of interspecies transmission has been suggested. This article examines the molecular epidemiology and the biology of PBV and highlights the major challenges in the field.