Most sinus infections are viral, and only a small proportion develops a secondary bacterial infection. Rhinoviruses, influenza viruses, and parainfluenza viruses are the most common causes of sinusitis. The most common bacteria isolated from pediatric and adult patients with community-acquired acute purulent sinusitis are Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and Streptococcus pyogenes. Staphylococcus aureus and anaerobic bacteria (Prevotella and Porphyromonas, Fusobacterium and Peptostreptococcus spp.) are the main isolates in chronic sinusitis. Pseudomonas aeruginosa and other aerobic and facultative gram-negative rods are commonly isolated from patients with nosocomial sinusitis, the immunocompromised host, those with HIV infection, and in cystic fibrosis. Fungi and Pseudomonas aeruginosa are the most common isolates in neutropenic patients. The microbiology of sinusitis is influenced by the previous antimicrobial therapy, vaccinations, and the presence of normal flora capable of interfering with the growth of pathogens.