A highly contagious virus infection of horses, influenza is the single most important equine respiratory disease in many countries. Two subtypes of equine influenza virus have been identified, A/equine-1 and A/equine-2, neither of which immunologically cross-reacts. In the case of A/equine-2 virus, two lineages exist, American and European, which appear to have evolved independently of one another. The acute febrile respiratory disease characteristic of influenza is frequently complicated by secondary bacterial infection, especially in unvaccinated horses. Primarily a respiratory-borne infection, influenza has been spread to a significant number of countries through the international movement of horses. Strains of A/equine-2 virus have been responsible for all known outbreaks of the disease since 1980. Simple rapid procedures are now available for the diagnosis of equine influenza. Prevention and control of influenza is based on frequent use of inactivated, adjuvanted vaccines, which confer only incomplete and short-term protection against this disease. To be maximally effective, vaccines need to be periodically updated and include influenza virus strains closely related to those in current circulation.