Through examples of data from three empirical design studies, this paper presents the type of analysis which cognitive psychology makes of the mental activities involved in design. These activities are analyzed at three levels: the way in which designers organize their activity, the main strategies they adopt, and the problem-solving processes they use. Different types of design tasks are presented: functional specification, software design and composite-structure design tasks. The relevance of the results for A.I. is discussed from an assistance viewpoint.