A survey is given of the work and life of Cornelis den Hartog up to the date in 1996 at which he retired from his position as a professor at the University of Nijmegen. Cornelis (Kees) den Hartog made important contributions to aquatic ecology in the widest sense,e.g. on brackish water typology, meiofauna (microturbellaria), macroinvertebrates, littoral algae, seagrasses and aquatic macrophytes. He favoured the ecosystem approach in aquatic ecology by studying structure and functioning in an integrated way. He lead 31 students to their doctor's degree (PhD).