The aim of this thesis is to ask, and attempt to answer, some pertinent questions about that type of psychological explanation which proceeds by simulation, or model building. The method chosen is a detailed examination of some models, mostly 18th and 19th century mechanical ones, together with a theoretically motivated discussion of the relations between these models and the development of psychological theories contemporary with them. Two types of model, formal and intimate, are distinguished, both by their aetiology and by the way they are used by working scientists, and several examples of each type are subjected to scrutiny, as are the intentions of their modellers in building or adopting them. Four main foci of interest emerge: the history of experimental psychology (the myth that experimental psychology was born circa 1870 is exploded); the sociology of science (the impact of developing technology on psychological theory, via the proffering of models, is clearly demonstrated); the philosophy of psychology (issues such as the nature of explanation and the problem of representation are dis¬ cussed); and, last but not least, theoretical psychology (the value of work in cognitive simulation, and of some work in Artificial Intelligence, is stressed and, partly, explained).