Aquatic biofilms, which are well-organized communities of microorganisms,are widespread in nature. They constitute a major problem in many environmental, industrial and medicalsettings. The use of advanced techniques has revealed biofilm structure, formation and ecology. Specialattention was given to the build-up of biofilm in dental unit waterlines (DUWLs), which are small-boreflexible plastic tubing to bring water to different handpieces. They are coated with well-establishedbiofilms. Active biofilm is a source of microbial contamination of DUWLs water. The safety of dentaltreatment requires a good quality of the water used. The knowledge of nature, formation and the waysto eliminate the biofilm is the first step towards reducing health risk, both for patients and dentalpersonnel. The article reviews these issues.