The psychological literature relating to factors which should be taken into account in the design of illustrations and pictorial instructions as aids to problem solving is reviewed. The problem solver can invoke a wide range of strategies for using pictorial materials to achieve his goal, although pictorial instructions which indicate how to perform actions place a tighter constraint on the choice of strategy than illustrations showing only states to be attained. The designer must make his graphics compatible with as wide a range of strategies for using them as possible while building on the knowledge the problem solver already possesses. "Problem Solving" is a term which has been used very freely to describe any cognitive activity for which the goal but not the means for obtaining it are initially specified. The problem solver has a more or less well defined set of operations which he can perform to change the problem situation, with the objective of eventually reaching a state which he recognises as a solution (Simon and Hayes, 1976). The choice of operations to be performed is likely to be governed by some kind of strategy, which may indicate desirable intermediate positions to be attained; what is done at each stage, however, will depend on the outcome of previous steps and the progress they have brought towards the intermediate or ultimate solution. Problem solving encompasses a wide range of activities, and the ways in which pictorial materials can help are equally varied. They can be used to answer specific questions arising during the course of problem solving activities, or provide specific items of information, as Barnard and Marcel (1978) observed, and they can influence the type of problem solving strategy adopted which will, in its turn, determine what specific information is sought (Szlichcinski, 1978). Indeed, when the instructions for performing a task are presented in pictorial form, understanding the instructions to perform the task becomes a complete problem solving activity in itself (Simon and Hayes, 1976), an interactive process in which the user makes a series of references to the instructions, the information sought depending on the outcome, successful or unsuccessful, of previous consultations.