[Basic Approach : Transsphenoidal Approach (Nasal and Sphenoid Phase)].

Endoscopic endonasal transsphenoidal surgery has become a cornerstone of minimally invasive skull base surgery, particularly for pituitary neuroendocrine tumors. Advances in endoscopic visualization, navigation systems, and surgical instruments have enabled safe and effective use of a direct transnasal corridor to the sella and adjacent skull base structures. However, successful execution of this approach requires a precise understanding of nasal and paranasal sinus anatomy, as well as meticulous surgical technique during the nasal and sphenoid phases of the procedure. This chapter describes the basic concepts and step-by-step techniques of the endoscopic endonasal transsphenoidal approach, with a special emphasis on nasal cavity expansion, sphenoid sinus access, and sellar floor opening. Key anatomical landmarks, including the lamellar concept of the ethmoid sinus, the sphenoid sinus natural ostium, and the relationship between the internal carotid artery and surrounding structures, are highlighted to enhance intraoperative orientation and safety. Practical guidance on mucosal incision, septal cartilage preservation, bone removal, and reconstruction strategies to prevent cerebrospinal fluid leakage is also provided. This chapter aims to establish a practical foundation for novice and intermediate surgeons, facilitating a safe transition to more advanced endoscopic skull base procedures through systematic anatomical understanding and standardized surgical techniques.