Gertjan Medema is chief science officer and principal microbiologist. He is also part-time chair on Water & Health at Delft University of Technology. His research focusses on understanding of the (barriers against) transmission of infectious diseases via water systems as scientific basis for the design of safe water systems. This encompasses: (1) development of methods for detection/typing/source tracking of pathogens in water and soil; (2) study the occurrence, fate and transport of pathogens in water systems (wastewater, surface water, groundwater, drinking water, urban water systems, water reuse systems, aerosols) (experimental & modelling); (3) assessment of the removal of micro-organisms by water treatment processes; (4) integrating this knowledge in global/national water-pathogen models and into quantitative risk assessment/health impact assessment; (5) design of safe water systems and advise on effective risk management. As chief science officer he coordinates the joint research programme of the Dutch water utilities (BTO) and the research strategy and environment at KWR. He is an internationally recognized expert and scientific coordinator of the WHO collaborating centre on Water Quality and Health, active in the WHO working group on microbiology; advisor of EU DG Environment on water reuse guidelines, IWA fellow and (past) chair of the specialist group on Health-Related Water Microbiology; member of the Dutch Health Council Committee on safe bathing water.