The architecture of the lung is discussed with special focus on lung parenchyma. The lung parenchyma is mainly comprised of numerous air-containing passages and intervening fine structures, corresponding to alveolar ductal lumens and alveoli, as well as alveolar septa and small pulmonary vessels occupying 10% of total parenchymal volume. The shapes and relative arrangements of alveolar ducts and alveoli are discussed in detail, which is followed by a brief description of bronchial circulation and pulmonary lymphatics.