Bacterial biofilms are three-dim ensional aggregates of bacteria encased in secreted exopolysaccharides (slime) and lack the susceptibility to antibiotics that planktonic bacteria demonstrate. Bacterial biofilms are present in the sinonasal mucosa of some patients with chronic rhinosinusitis. The vast majority of bacteria exist in biofilms. Bacterial biofilms are a likely contributing factor to medically recalcitrant CRS. Strategies developed to treat planktonic bacteria are ineffective against bacteria in a biofilm. Understanding how biofilm infections form is fundamental to developing rational strategies for prevention and treatment. © 2009 Springer Berlin Heidelberg.