Most of the research on bacterial pathogenesis has focused on acute infections, but much less is known about the pathogenesis of infections caused by bacteria that grow as aggregates in biofilms. These infections tend to be chronic as they resist innate and adaptive immune defence mechanisms as well as antibiotics, and the treatment of biofilm infections presents a considerable unmet clinical need. To date, there are no drugs that specifically target bacteria in biofilms; however, several approaches are in early-stage development. Here, we review current insights into biofilm physiology and pathology, and discuss how a deep insight into the physical and biological characteristics of biofilms can inform therapeutic strategies and molecular targets for the development of anti-biofilm drugs.