Moraxella catarrhalis is an exclusively human pathogen and is a common cause of otitis media in infants and children, causing 15%-20% of acute otitis media episodes. M. catarrhalis causes an estimated 2-4 million exacerbations of chronic obstructive pulmonary disease in adults annually in the United States. M. catarrhalis resembles commensal Neisseria species in culture and, thus, may be overlooked in samples from the human respiratory tract. The prevalence of colonization of the upper respiratory tract is high in infants and children but decreases substantially in adulthood. Most strains produce beta-lactamase and are thus resistant to ampicillin but susceptible to several classes of oral antimicrobial agents. Recent work has elucidated mechanisms of pathogenesis and focused on vaccine development to prevent otitis media in children and respiratory tract infections caused by M. catarrhalis in adults with chronic obstructive pulmonary disease.