Sinusitis generally develops as a complication of viral or allergic inflammation of the upper respiratory tract. The bacterial pathogens in acute sinusitis are Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis, while anaerobic bacteria and Staphylococcus aureus are predominant in chronic sinusitis. Pseudomonas aeruginosa has emerged as a potential pathogen in immunocompromised patients and in those who have nasal tubes or catheters, or are intubated. Many of these organisms recovered from sinusitis became resistant to penicillins either through the production of beta-lactamase (H. influenzae, M. catarrhalis, S. aureus, Fusobacterium spp., and Prevotella spp) or through changes in the penicillin-binding protein (S. pneumoniae). The pathogenicity of beta-lactamase-producing bacteria is expressed directly through their ability to cause infections, and indirectly through the production of betalactamase. The indirect pathogenicity is conveyed not only by surviving penicillin therapy, but also by 'shielding' penicillin-susceptible pathogens from the drug. The direct and indirect virulent characteristics of these bacteria require the administration of appropriate antimicrobial therapy directed against all pathogens in mixed infections. The antimicrobials that are the most effective in management of acute sinusitis are amoxycillin-clavulanate (given in a high dose), the newer quinolones (gatifloxacin, moxifloxacin) and the second generation cephalosporins (cefuroxime, cefpodoxime, cefprozil or cefdinir). The antimicrobials that are the most effective in management of chronic sinusitis are amoxycillinclavulanate, clindamycin and the combination of metronidazole and a penicillin.