Dirofilaria immitis is the etiologic agent for canine heartworm disease. Although it is most notably associated with disease in dogs, various other species can serve as the definitive hosts. When left unchecked, infection can be fatal. Heartworm disease in cats is an interesting and sometimes frustrating condition to deal with as it is difficult to diagnose. The true prevalence of this disease in felines is unknown. It is believed to be less than that in dogs in the same endemic areas. This paper is intended to review heartworm disease as it relates to cats with respect to prevalence, clinical signs, diagnosis, and treatment.