This chapter discusses complications in endoscopic sinus surgery, where close endoscopic observation and course checking throughout the surgery provide safe surgery and immediate recognition of problems with solutions. Successful endoscopic sinus surgery (ESS) requires adequate preoperative planning including medical therapy, endoscopic examination, and thorough review of radiographic studies, computed tomography (CT) scans, and occasionally magnetic resonance imaging (MRI) studies. The middle turbinate is a guide to the antrostomy and the maxillary line. The horizontal basal lamella of the middle turbinate receives the blood supply from the sphenopalatine foramen. One of the most confusing and treacherous area to open in ESS is the ethmoid skull base. Traditional or endoscopic septoplasty is very helpful toward safe visualization. Any mental status change, severe headache, or unilateral clear liquid drainage is consistent with possible skull base injury and needs radiologic CT evaluation. When the eye muscle is injured, double vision, diplopia, occurs.