There is a very extensive experimental literature on the broad subject of psychomotor skills. The usual starting point is Bryan and Harter’s work on morse telegraphy in 1897. This literature is summarised in most basic texts (e.g. Hovland 1951, Woodworth and Scholeberg 1955) and it is not proposed to repeat their summaries here. What is intended is to cover that part of the literature which bears directly on differential transfer during the course of practice at a psychomotor task. By “differential transfer” is meant the changes in the pattern of abilities transferring to the task, which seem to occur as performance improves.