The investigation was based on a sample of 1,425 London boys aged 13 to 16 years who were questioned regarding their degree of involvement in stealing. A complete description covers the development of hypotheses for the research and the construction of a technique for eliciting information from boys about their involvement in stealing. The intensive interview, the progressive modification method for eliciting information, and examples of the questions asked are considered. Attention is given to the sampling methods used and the methodological problems in discovering an accurate sample without full listings being available. Finally, several findings are given that relate to the nature and extent of stealing by London boys. Next, the strategy used for investigating causal hypotheses is outlined, with particular emphasis on the Hypothetico-Deductive Method. This method involves the construction of a hypothesis that is not directly testable because its operation is not controllable. The propositions (called expectations) that are deduced from this hypothesis are explained, along with the method for testing them and the way by which these propositions can support an untestable hypothesis. Diagrams further illustrate the method. Tables and illustrations are included.