Heartworm disease is a common condition in dogs living in climates capable of supporting mosquito populations and transmission of Dirofilaria immitis. Dogs become infected with stage three larvae when they are bitten by an infected mosquito. Over the next five to seven months, these larvae migrate in the subcutaneous tissues and molt several times before migrating to the pulmonary arteries. Here the worms mature into adults capable of producing microfilaria and active infection. Adult worms can live as long as seven years in the pulmonary vasculature, leading to vasculature damage and right-sided heart enlargement. In the worst cases, heartworm disease may progress to caval syndrome characterized by right sided heart failure, pulmonary hypertension, anemia, disseminated intravascular coagulation, and death. Antigen tests are both sensitive and specific but only detect adult worms, and thus false negative results can be obtained when a dog is tested during the prepatent period. For this reason, heartworm disease can go undetected for a prolonged period of time when dogs from endemic areas that have not been receiving monthly preventive treatments are tested a single time. This paper will describe heartworm infection in an individual dog and emphasize the pathogenesis, testing, and treatment of this common yet preventable condition.