Influenza C virus (ICV) was discovered in 1947 and detected in humans, with natural infections occurring periodically. However, early studies on ICV were challenging in diagnosis because the virus is difficult to culture. As a result, the disease burden and pathogenicity of ICV have been underestimated. Recent studies using molecular biological techniques such as real-time polymerase chain reaction have provided further insights into prevalence, seasonality, genomic diversity, and evolution. In addition, the possibility of interspecies transmission was suggested based on the high similarity between the nucleotide sequence of ICV confirmed to infect animals and the sequence of ICV isolated from humans. In this review, we summarize current data on the epidemiology and clinical features, viral genome analysis, and animal studies of ICV.