Interpretations of human careers vary with moral beliefs and political ideologies. Attempts to measure both the quantity and quality of criminal activities are subject to errors or bias in data collection, scoring, and analysis. For example, identifying variables to be measured depends on the clarity with which events are initially defined. There are many roads to a career; human action does not result from a single cause. Individual constitutions (differences in cognitive style, intelligence, temperament, and motivation) can contribute to criminal careers, but there is more to career than constitution. Individuals with a common criminal career may share personality features, but the proportion of people with such personalities who follow criminal careers is unknown. Behavior can also be explained by exploring how learning occurs and how its content influences actions. Another cause of behavior -- environment (i.e., work and power) -- influences human action but is difficult to isolate from other causes. In sum, it is possible to know something about the causes of conduct without knowing enough to make accurate predictions. The text contains about 850 references, tables, a subject and name index, and a separate booklet of test and discussion questions. It can be used singly or in conjunction with the three accompanying volumes on criminal careers. For the other volumes, see NCJ 87529-31.