The distinction between 'knowing how' and 'knowing that' is fundamental to current theories of cognition. Two distinct encodings or representations are implied, one conscious and verbalisable and the other normally unconscious yet demonstrable in behaviour. The paper discusses the nature of these two kinds of representation and relations between them. It is shown that imagery forms an essential mediating link between the two encodings and a theoretical model-the Action-Language-Imagination or ALI model-is presented. An important feature of the model is the role attributed to the motor system in generating imagery and principal features of motor imagery are reviewed in the context of the ALI model and with reference to recent experimental findings. Problems in mapping conscious representations of action onto physical brain mechanisms are briefly discussed. It is proposed that the physical basis of imaginal representations of actions is best understood in terms of the mechanisms of motor control. A two stage theory of motor imagery is proposed in which the first stage, the generation of a prototypical action is virtually identical to that involved in overt actions whilst the second stage depends on the retrieval of sensory impressions from memory.