Most sinus infections are viral and only a small percentage develop bacterial infection. Rhino-, influenza, and para-influenza viruses are the most frequent viral causes of sinusitis. The most common bacterial isolates from children and adult patients with community-acquired acute bacterial sinusitis are Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and Streptococcus pyogenes. Staphylococcus aureus and anaerobic organisms (Prevotella and Porphyromonas, Fusobacterium, and Peptostreptococcus spp.) are the commonest isolates in chronic rhinosinusitis (CRS). Aerobic and anaerobic beta lactamase-producing bacteria (BLPB) were recovered from over a third of these patients. Methicillin-resistant S. aureus (MRSA) accounted for over 60 % of S. aureus isolates. Pseudomonas aeruginosa and other aerobic and facultative Gram-negative rods are frequently recovered in nosocomial sinusitis, the immunocompromised host, individuals with human immunodeficiency virus infection, and in cystic fibrosis. The CRS infection evolves the formation of a biofilm that might play a significant role in the pathogenesis and persistence of CRS. The microbiology of sinusitis is influenced by previous antimicrobial therapy, vaccinations, and the presence of normal flora capable of interfering with the growth of pathogens. Recognition of the unique microbiology of CRS and their antimicrobial susceptibility is of great importance when selecting antimicrobial therapy.