An overview of the historical development of risk and need assessment in the United Kingdom focuses on probation practice in the mid-1990's; the "What Works" movement that gained momentum during this period; the development of the Level of Service Inventory - Revised (LSI-R); the development and introduction of ACE (assessment, case management, and evaluation); and the development and introduction of the Home Office's own assessment system ("OASys"). In considering the validity and value of the assessment tools that have been developed and used in the United Kingdom, this chapter addresses their value in assessing need, in assessing the risk of reconviction, in assessing the risk of serious harm, in matching service levels to risk, in supervision planning and targeting interventions, in improving offender engagement, and in measuring change. Also discussed is the value of assessment tools for practitioners, managers, and researchers. In discussing the future for assessment tools in probation, the author advises that the OASys, as the future assessment system for probation and prisons, must be presented to practitioners and managers in terms of its proven capabilities and limitations. Gaps in its performance must be filled by future research, particularly its ability to measure change. 1 figure, 6 tables, and 22 references