Traditional waterborne infections have been largely controlled in the UK by the provision of clean drinking water. However, water can still cause problems for infection control teams in particular when used in endoscope washer-disinfectors. HTM 2030 states that final rinse water used in washer-disinfectors must not present a microbiological hazard and that there should be no recovery of micro-organisms from the final rinse water. The problems that biofilms may cause in washer-disinfectors, the type of biofilms that may develop, and the nature of the bacteria within them, in particular how biofilm bacteria behave differently to those that are not part of a biofilm (planktonic bacteria), are discussed in this article. Finally, we discuss how knowledge of the growth and control of biofilms may be used to control their growth.