Rhinosinusitis is defined as inflammation of one or more of the paranasal sinuses and affects ∼16% of the population. Acute rhinosinusitis is defined as symptoms lasting <4 weeks and subacute rhinosinusitis is between 4 and 8 weeks. Chronic rhinosinusitis (CRS) is defined as symptoms lasting >8-12 weeks. CRS is divided into three groups: CRS with nasal polyps, CRS without nasal polyps, and allergic fungal rhinosinusitis. The sinus cavities are lined with pseudostratified ciliated columnar epithelial cells interspersed with mucous goblet cells. Cilia continuously sweep the mucous toward the ostial openings and are important in maintaining the proper environment of the sinus cavities. The frontal, maxillary, and anterior ethmoid sinuses drain into the ostiomeatal unit of the middle meatus. The posterior ethmoid sinuses and superior sphenoid sinuses drain into the sphenoethmoid recess of the superior meatus. Most acute sinus infections are caused by viruses and, therefore, it is not surprising that the majority of patients improve within in 2 weeks without antibiotic treatment. A bacterial infection should be considered if symptoms worsen or fail to improve within 7-10 days. Amoxicillin, trimethoprim-sulfamethoxazole, or doxycycline are first-line therapy. The Joint Task Force on Practice Parameters for Allergy and Immunology suggests assessing response to symptoms after 3-5 days of therapy and continuing for an additional 7 days if there is improvement. Combining an intranasal corticosteroid with an antibiotic reduces symptoms more effectively than antibiotics alone.