The relationship between pulmonary hypertension and left heart disease is complex. When initial assessment suggests the presence of pulmonary hypertension, it is critical to determine its precise nature: pulmonary arterial hypertension versus pulmonary hypertension owing to left heart disease. Clues to diagnosis can be found in the history, physical examination, initial laboratory testing, and imaging studies. Treatment requires optimal therapy of the underlying left heart disease. It is uncertain whether therapies for pulmonary arterial hypertension have any role in treating pulmonary hypertension owing to left heart disease, and there are safety concerns with these agents in this population.