To successfully make the transition from school to adult life and the world of work, adolescents and young adults need guidance and encouragement from caring, supportive adults. The best decisions and choices made by transitioning youth are based on sound information including appropriate assessments that focus on the talents, knowledge, skills, interests, values, and aptitudes of each individual. This guide will serve as a resource for multiple audiences within the workforce development system. It will be useful to policymakers, administrators, and a wide variety of youth service practitioners including teachers, transition coordinators, counselors, work experience coordinators, tutors, job placement specialists, and job coaches. The guide can be used in many settings including secondary and postsecondary education programs, school-to-work transition programs, One-Stop workforce centers, youth employment programs, community rehabilitation programs, and community-based organizations that serve youth and young adults, ages 14 to 25. For youth service practitioners, this guide will: (1) describe the purposes and dynamics of four ways to assess; (2) delineate how to select and use assessment tools, both formal and informal; (3) provide practical information about many commonly used published assessment and testing instruments; (4) describe when and how to seek help or further information about assessments; and (5) review legal issues, ethical considerations, and confidentiality as they pertain to assessment and testing. For administrators and policymakers, this guide will provide helpful information in developing: (1) practical and effective policies; (2) greater collaboration among programs; and (3) interagency assessment systems. Overall, using this guide in transition planning programs can improve: (1) the quality of services at the local level; (2) strategic planning at state and local levels; and (3) results for youth. Appended are: (1) Assessments and Federal Laws; and (2) Glossary of Terms Used in this Guide. Individual chapters contain references. (Contains 3 figures, 7 tables and 10 exhibits.) [For the previous edition, "Career Planning Begins with Assessment: A Guide for Professionals Serving Youth with Educational and Career Development Challenges," see ED485703.]