This paper provides a definition of assessment that is tailored to the unique considerations of the concept and practice of the transition of students with disabilities from school to adult life. Assessment in transition is considered to be the on-going process of obtaining and properly using both person and environment-based assessment information for the purpose of aiding in decision making about individualized program planning and implementation of needed educational, vocational, and community-based services for youth with disabilities in transition. The need for a collaborative and systematic model program through which assessment activities should be performed is discussed. Such a program would help to determine what type of information is to be gathered, who is responsible for gathering it, when and how often it is to be gathered, who is responsible for recording and interpreting it, how it is to be used, and who will be the users. (Contains 18 references.) (JDD)