Bleeding from the lung originates from the bronchial vessels, the pulmonary vessels, or the microcirculation of the lung. Bleeding of bronchial origin is usually a result of bronchiectasis or endobronchial malignancy. Pulmonary hemorrhage originating from the small, medium, and large pulmonary vessels is most commonly due to systemic vasculitis, which can also involve the microcirculation. Vasculitides involving the microvasculature is known as pulmonary capillaritis. Diffuse alveolar hemorrhage (DAH) is a clinicopathologic syndrome describing the accumulation of intraalveolar RBCs originating from the alveolar capillaries. All causes of DAH have the common denominator of an injury to the alveolar microcirculation. The clinical syndrome includes hemoptysis, anemia, diffuse radiographic pulmonary infiltrates, and hypoxemic respiratory failure, which can be severe. DAH is associated with a number of clinical entities and several histologic subtypes. The most common underlying histology of DAH is of a small vessel vasculitis known as pulmonary capillaritis, usually seen with seropositive systemic vasculitides, or a connective tissue disorder, bland pulmonary hemorrhage, and diffuse alveolar damage due to a number of injuries including drugs, coagulation disorders, and infections. Pulmonary capillaritis may also be a small vessel vasculitis limited to the lung. BAL confirms the clinical diagnosis of DAH; however, a surgical lung biopsy may be required to confirm the underlying histology. In general, surgical lung biopsy is considered if DAH is associated with negative serology and not a part of a systemic disease. Treatment is directed at the underlying diagnosis and typically includes corticosteroids, immunosuppressive agents, and occasionally plasmapheresis.