This article presents a training course in work analysis via an understanding of real work. The course was aimed at trade unionists and was produced by researchers in ergonomics and occupational medicine. Designing this type of training is closely associated with the history of ergonomics in France and goes back to the basic principles: training by and for action, focusing on the concept of activity and the co-construction of knowledge, involving all actors in the relevant area of work. One hundred and fifty trade unionists were trained over a period of 18 months, successfully completing company projects in the field of occupational health. The course was organised into seven 3-day modules. Each module included plenary sessions on fundamental topics relating to health/work, and small work groups where the trade union projects were prepared, based on gaining an understanding of the real work involved. There were two types of results: an understanding of the way work is currently changing and of the consequences of these changes for workers' health; transformations in trade unionists' representations of work, giving them a better awareness of health/work relationships, new ways of transforming work, and also information about the teaching content required when designing training programmes for trade unionists. Understanding the notion of activity and constructing interaction with employees are crucial elements for carrying out this type of project successfully.