Bacteria that attach to a surface and grow as a biofilm are protected from killing by antibiotics. Reduced antibiotic susceptibility contributes to the persistence of biofilm infections such as those associated with implanted devices. The protective mechanisms at work in biofilms appear to be distinct from those that are responsible for conventional antibiotic resistance. In biofilms, poor antibiotic penetration, nutrient limitation and slow growth, adaptive stress responses, and formation of persister cells are hypothesized to constitute a multi-layered defense. The genetic and biochemical details of these biofilm defenses are only now beginning to emerge. Each gene and gene product contributing to this resistance may be a target for the development of new chemotherapeutic agents. Disabling biofilm resistance may enhance the ability of existing antibiotics to clear infections involving biofilms that are refractory to current treatments.