An introduction is given to the nature of information processing models in psychology. It is then claimed that, so far as cognitive and linguistic processes are concerned, there are no practical constraints from biology into psychological theory. Exceptions to this principle may be found where there is a precise one—one mapping of psychological and biological units. However, the study of the effects of brain damage makes it implausible that such relations exist for higher mental functions. Finally the possibility of relations between psychological and linguistic descriptions is explored by examining the results of experiments on the influence of word morphology in a number of tasks. It is concluded that psychology and linguistics do not constrain each other, but might exchange useful pointers to possible advances.