Manufacturers of manually assembled products are increasingly being forced to improve the flow of assembly orders, together with having more efficient employment, in order to meet increasing demands on short delivery times, high product variety, good quality and low manufacturing costs. At the same time, the human factor is increasingly important-a well-motivated workforce is a necessity these days. To improve assembly processes, layout and workstation design, a participatory approach was developed by TNO and applied in various assembly companies. The goals of this approach are the involvement of assembly staff, lead time reduction and an improvement of the assembly tasks in an ergonomic sense. The main elements of the approach are the active participation of the company, the integration of two basic disciplines: assembly engineering and ergonomics, and its stepwise nature. In March 2000, the European project PSIM (Participative Simulation environment for Integral Manufacturing enterprise renewal) was launched. The goal of the project, in which the employees play a central role, is to enable the ongoing and integral improvement of company assembly processes using advanced simulation software. To support this participatory and integral approach to improve production lines, a software tool, the ERGOtool, is developed within the PSIM project. ERGOtool is a software tool that helps in the description and evaluation of current and future assembly processes. It takes factors such as physical load, ergonomic hazards, mental load, and processe flow characteristics into consideration.