A continuous changing environment imposes new requirements for assembly management and the role of human factors. To support an adequate response to these changes the European PSIM project was started in March 2000. This project aims at enabling the ongoing and integral improvement of assembly processes using advanced simulation software and up-to-date knowledge of sociotechnical systems design and ergonomics. The focus of this paper is on the development of the Ergonomic/Sociotechnical (E/S) software tool. Attention is paid to procedural aspects, the test of the first paper and pencil version and the development of the software prototype.