Equine influenza virus is a highly contagious agent that is capable of causing explosive outbreaks of respiratory disease among susceptible horse populations. The virus infects the mucosal cells of the upper and lower respiratory tract, and induces respiratory disease characterized by a harsh dry cough and fever. Infection is generally self-limiting and the majority of horses recover uneventfully; however, the recovery period may take several weeks to months. Outbreaks affecting performance horses exert a significant economic impact on the equine industry due to loss of performance and time out of work. Vaccination and careful management can limit the spread and severity of disease among groups of horses, but, in the past, vaccines have often failed to provide adequate protection. Vaccine failure has been attributed to genetic and antigenic drift of the influenza A/equine/2 virus from vaccine strains, as well as failure of some vaccines to stimulate the appropriate array of immune responses. This article offers the equine veterinarian a practical approach to preventing infection and the steps that can be taken to promote rapid diagnosis and effective control of outbreaks if preventive measures fail. Epidemiology, pathophysiology, clinical presentation, treatment, vaccines, and vaccination protocols are discussed, and a brief discussion of future technologies is presented.