Sublobar resection, which consists of segmentectomy and wedge resection, for the radical treatment of small-sized non-small cell lung cancer (NSCLC) in patients able to withstand lobectomy remains controversial. The dogma that lobectomy is the standard of care for NSCLC has been upheld until recently, although several current investigations, mostly from Japan, found equivalent outcomes of sublobar resection and lobectomy for T1a NSCLC. The current understanding of radical segmentectomy can be summarized as follows. First, the indications for segmentectomy should be limited to T1 tumors < or = 3cm in diameter, and high-resolution computed tomography (CT) and positron emission tomography-CT findings must be taken into consideration, particularly for T1b tumors. Whenever nodal involvement or an insufficient margin is confirmed intraoperatively, segmentectomy should be converted to lobectomy with complete nodal dissection. Second, radical and compromising indications for segmentectomy must be independently discussed. The former is for low-risk patients who can tolerate lobectomy. Third, segmentectomy is more valuable than wedge resection from an oncological perspective because it allows nodal dissection at the hilum. Thus, the choice of the most suitable procedure, such as whether to convert intraoperatively to lobectomy, should consider precise staging and the lower rate of local recurrence resulting from sufficient surgical margins. Therefore, segmentectomy must be clearly separated from wedge resection among the categories of sublobar resection for lung cancer. Surgeons must become adept at and master segmentectomy as a keynote procedure because small lung cancers are being detected with increasing frequency.