This chapter reviews first the measurement instruments and then describes the data-reduction procedures leading to the definition of key constructs. Because many constructs contain items from more than one measure, constructs are discussed after all measures have been described. Measures and constructs derive from two assessments: (a) the screening assessment (S), administered in the spring of 1987 for Cohort 1 and in the spring of 1988 for Cohort 2; and (b) the follow-up, followed for each cohort 6 months later. The followup was more extensive. From now on, the two cohorts will not be discussed separately, but will be combined. Official records of offending were searched in the Juvenile Court of Allegheny County in Pittsburgh. They covered up to the date of the follow-up for participants living in that county and, prospectively, for about 6 years subsequent to the follow-up assessment.