Equine coronavirus (ECoV) is an emerging virus associated clinically and epidemiologically with fever, depression, anorexia, and less frequently colic or diarrhea in adult horses. Sporadic cases and outbreaks have been reported with increased frequency since 2010 from Japan, the USA, and more recently from Europe. A feco-oral transmission route is suspected, and clinical or asymptomatic infected horses appear to be responsible for direct and indirect transmission of ECoV. A presumptive clinical diagnosis of ECoV infection may be suggested by clinical presentation and hematological abnormalities such as leukopenia due to lymphopenia and/or neutropenia. Confirmation of ECoV infection is provided by specific ECoV nucleic acid detection in feces by quantitative PCR or demonstration of coronavirus antigen by immunohistochemistry or electron microscopy in intestinal biopsy material obtained ante- or postmortem. The disease is generally self-limiting and horses typically recover with symptomatic supportive care. Complications associated with disruption of the gastrointestinal barrier have been reported in some infected horses and include endotoxemia, septicemia, and hyperammonemia-associated encephalopathy. This chapter reviews current knowledge concerning the etiology, epidemiology, clinical signs, diagnosis, pathology, treatment, and prevention of ECoV infection in adult horses.