A technical study, carried out in four plants and over a period of several years, to determine the psychological as well as physical causes of work curves. One hundred workers were analyzed, their homes and outside interests explored, their individual records studied (with their own close cooperation) in order to see what made varying patterns of ""zest for work"". This, the major part of the survey, will be an important contribution to industrial psychology, but the average reader will not be concerned. Plant managers, personnel directors, safety engineers-all the key people working with human industrial problems will find some phases of more interest than others. Almost anyone- in or out of the field- and still involved in human relations problems, will discover points of contact in the final sections which deal with the importance of the relations between foreman and men.