Early in 1985, a meeting was held to discuss the organization of a work shop in honour of Professor Reinier L. Zielhuis on the occasion of his retirement on November 30,1986. Various themes for such a workshop were considered, but it was Zielhuis himself, who ultimately selected "Health surveillance of individual workers exposed to chemical agents" as the theme of the workshop. Although this topic is of the utmost im portance for occupational health practice, it is also a very difficult one, given our currently limited knowledge on the subject. The choice of this topic is characteristic of Reinier Zielhuis, a scientist and scholar who throughout his career has been deeply concerned with the health and health risks of workers. Because he regards occupational and environmental exposure - including life style - as two aspects of the total exposure to chemical and physical factors, he emphasizes the need to assess the impact of this total exposure on workers' health. The impact of total exposure probably detennines to a large extent the in dividual variability in response. This was the main reason why he selected the above-mentioned topic for the workshop.