Although the normal pulmonary vascular transition at birth takes place quickly in the delivery room, it has its basis in the complex structural and biochemical development of the lung. It is not surprising that this process can be easily disrupted by factors such as prematurity, intrauterine hypoxia, and parenchymal lung disease. The stimuli that initiate and maintain the transition are only beginning to be understood. Understanding the normal structure and function of the neonatal lung provides the foundation that will enable clinicians to enhance resuscitation to accomplish a normal transition in the delivery room.