This lecture reviews the endoscopic anatomy of the skull base beginning at the frontal, ethmoid, and sphenoid sinuses and extending across the sella and down the clivus to the foramen magnum. The anatomy of the area is reviewed from both above and below with a focus on the endoscopic anatomy of the transnasal approaches to the anterior fossa, sella, clivus, cavernous sinus, and medial part of the CP angle.