Because existing experimental measures of vocational maturity were too narrow or difficult to administer and score, the Career Development Inventory (CDI) was developed to evaluate the effects of a computer-based vocational guidance system on the vocational development of high school students, and early research suggests that the CDI promises to be an important aid in research and counseling for vocational development. The CDI consists of three factor-analytic scales, including: (1) Planning Orientation, which represents the degree of the student's awareness toward planning and choice, (2) Resources for Exploration, which assesses the quality or soundness of individually used and potentially available resources, and (3) Information and Decision Making, which samples the amount of educational and occupational information and the use of the information for sound decisions. Test-retest reliabilities for the three scales over a short period of time were above .70, and reliabilities over a 6-month period were above .60, indicating that vocational maturity was a stable characteristic. Tentative results from content validity, criterion-related concurrent and predictive validity, and construct validity studies indicate that the CDI does measure important aspects of vocational maturity. Uses of the instrument for program evaluation, group assessment, and individual counseling are suggested. (SB)