The function of the remaining lungs after major lung resection may be a determinant of the early postoperative outcome, as well as the late postoperative quality of life of the patient. Thus, extensive efforts have been made to accurately estimate the postoperative pulmonary function using a variety of methods: the segment counting method is utilized in patients without parenchymal diseases, while the functional lung imaging technique may be useful in patients with heterogeneous anatomical lung diseases. The postoperative pulmonary function is influenced not only by the extent of parenchymal resection, but also by various other factors, such as the site of resection, the mode of thoracotomy, the severity of pulmonary emphysema and/or the postoperative progression of pulmonary fibrosis. Although thoracoscopic surgery or segmental resection can lessen the extent of chest wall damage or the extent of parenchymal resection compared with conventional operations, the resulting functional benefits do not last. Interestingly, the postoperative pulmonary function continues to improve during the first postoperative year as if the remaining lungs grow, although the cause(s) of this compensatory response of the remaining lungs remains unclear. Such an ability of the remaining lung to compensate for the lost lung function may eventually determine the late postoperative pulmonary function.