Acute sinusitis is one of the most common infections seen in general clinical practice. The most common cause of acute sinusitis is viral; however, many patients receive a prescription for an antibiotic. Such injudicious prescribing habits have a major impact on health care costs, contribute to the increasing prevalence of drug-resistant strains of common respiratory pathogens, and reflect many of the challenges in differentiating viral and bacterial disease. Sinus puncture and culture of the aspirate, the diagnostic reference standard in the research setting, are not appropriate for routine clinical practice. However, certain clinical signs and symptoms that do not improve or that worsen after 7-10 days are currently accepted criteria for diagnosis of bacterial sinusitis. Accurate diagnosis can select patients who would benefit most from antimicrobial use. Antimicrobial agents should be selected on the basis of local resistance patterns, and their spectrum of activity should cover the common bacterial pathogens, including resistant strains.