Understanding the anatomy of the paranasal sinuses is the most important prerequisite for performing safe, effective endoscopic sinus surgery (ESS). The most feared complications of ESS are the result of an uncontrolled maneuvering beyond the boundaries of the sinuses, mainly into the orbit or through the base of the skull, which are prevented through a thorough understanding of the relevant anatomy. However, even with the most experienced surgeons these complications can occur, facilitated by either preexisting anatomical variants, pathological modifications caused by the underlying disease, or both. In the course of developing ESS as a viable approach to the treatment of diseases of the paranasal sinuses, many constant landmarks were described by different authors since the technology was first introduced in the late 1970s. These anatomical landmarks allow beginners to achieve and maintain good spatial orientation when navigating within the sinuses, and help them perform the most complete sinus surgery while minimizing risk to the patient. However, there is definite learning curve as trainees progress through more difficult ESS procedures requiring increasing levels of surgical skills, starting with the simple nasal endoscopy and ending with the most advanced frontal sinus procedures. In addition to a detailed three-dimensional understanding of the anatomy, a thorough knowledge of the embryology, pathology, and imaging of the sinuses is also very important for the comprehensive management of sinus pathology, but these topics are beyond the scope of this chapter.