Personnel work, ranging from the initial selection of soldiers to the rehabilitation of combat casualties, was at the center of psychologists' wartime effort. It absorbed the energies of the largest fraction of the profession and involved a large number of psychologists with no previous experience in test design, construction, and administration. Their work addressed the fundamental task of sorting manpower.

In the great triumvirate of personnel procedures – selection, classification, training – the third task presented a distinct set of challenges. Although existing mental testing techniques were readily adapted for the selection and classification of soldiers, as we have seen, it was less clear how psychology might contribute to the development of effective training procedures. Experimental psychologists, many of whom had previously worked mainly with animals, took up the challenge and applied their knowledge about sensation, perception, and learning to the problems of training people.

Almost from the start of the war, experimental psychologists were involved in applied research on the psychological problems engendered by modern weapons technology. Consciously or not, such weapons were designed with human operators in mind, and the engineers responsible for building them had to rely on some notion of human physical and psychological capacities. Some weapons, such as rifles, had evolved over such long periods that their basic configuration was set and largely unproblematic for the average user. Other technologies, such as range finders and fire-control devices on large shipboard guns, were newer, and the fit between the hardware and its human handler was not always good.