An examination of the continuing debate among human service professionals about a precise definition of aptitude, with special attention devoted to the relationship of interests and intelligence to aptitude, is provided in this monograph. An historical look at aptitude test development is provided with a discussion of how individuals and events influenced the evolution of the tests. The General Aptitude Test Battery, a commonly used multiple aptitude test in rehabilitation, is analyzed to explain how multiple aptitude vocational tests were developed to provide a comprehensive evaluation of vocational potential. In addition, the practical utility of vocational aptitude tests in human services is addressed and the controversy regarding the use of general cognitive ability measures as opposed to specific aptitude tests to collect data and make placement decisions is examined. Finally, a discussion of the application issues involved in testing persons with disabilities is provided. (BHK)