This paper focuses on the use of knowledge possessed by designers. Data collection was based on observations (by the cognitive ergonomics researcher) and simultaneous verbalisations (by the designers) in empirical studies conducted in the context of industrial design projects. The contribution of this research is typical of cognitive ergonomics, in that it provides data on actual activities implemented by designers in their actual work situation (rather than on prescribed and/or idealised processes and methods). Data presented concern global strategies (the way in which designers actually organise their activity) and local strategies (reuse in design). Results from cognitive ergonomics and other research that challenges the way in which people are supposed to work with existing systems are generally not received warmly. Abundant corroboration of such results is required before industry may consider taking them into account. The opportunistic organisation of design activity is taken here as an example of this reluctance. The results concerning this aspect of design have been verified repeatedly, but only prototypes and experimental systems implementing some of the requirements formulated on their basis, are under development.