Although functional endoscopic sinus surgery is an effective means of treating patients with recurrent and refractory sinusitis, the procedure is not without risk of serious surgical complications. Preoperative computed tomography (CT) affords radiologists the opportunity to prospectively identify anatomic variants that predispose patients to major surgical complications; however, these critical variants are not consistently evaluated or documented on preoperative imaging reports. The purpose of this review is to illustrate important anatomic variants and landmarks on the preoperative sinus CT with a focus on those that predispose patients to surgical complications. These critical variants and landmarks can be quickly recalled and incorporated into the preoperative imaging report through the use of the mnemonic "CLOSE": Cribriform plate, Lamina papyracea, Onodi cell, Sphenoid sinus pneumatization, and (anterior) Ethmoidal artery. This approach will greatly enhance the value of the preoperative imaging report for referring otolaryngologists and help reduce the risk of surgical complications. (©) RSNA, 2016 Online supplemental material is available for this article.