It is more than a century ago that Moraxella catarrhalis was discovered and described in some detail. However, it was not until the last decade that M. catarrhalis was recognized as a facultative pathogen, namely in otitis media (predominantly in children), sinusitis and nosocomial pneumonia in the group of elderly, debilitated patients. Liberation of endotoxin, histamine, and chemotactically active factors can be considered the major pathogenicity factors. The pathogen can protect itself, on the one hand by binding of the Clq subcomponent of the complement system followed by subsequent formation of a functionally inactive complex with Cl, and on the other hand by inactivation of the terminal (lytic) complement complexes by means of a specific protein on the surface of the outer cell wall. Routine diagnostic procedures require, above all, culture of the pathogen: up to now the detection of specific IgA-antibodies has not been routinely available. More than half of the clinical isolates are known to exhibit beta-lactamase production (BRO-enzymes). This is the reason why combinations of a penicillin compound with a beta-lactamase inhibitor, the group of the newer cephalosporins (including the orally active ones), doxycycline and the macrolides are therapeutically effective.