The nasoseptal flap had an important role in the development of endoscopic endonasal surgery of the cranial base. The flap is pedicled upon the posterior septal artery, which is a terminal branch of the sphenopalatine artery. The reliable vascular supply promotes rapid healing and the flap is an effective barrier for the prevention of CSF leaks. For large skull base defects, it has dramatically decreased the risk of a postoperative CSF leak to less than 5%. The nasoseptal flap is a versatile flap with a wide arc of rotation that allows the flap to reach defects from the frontal sinus to the lower clivus. This chapter provides a comprehensive review of the nasoseptal flap. The harvesting and reconstruction techniques, modifications of the flap and complications are described.