In March 1999, in the context of its Effective Practice Initiative, the British Home Office announced its intention to develop a new risk/needs assessment instrument for use by all prisons and probation areas. At the time this article was written, the Offender Assessment System (OASys) was in the process of being introduced to probation and prison services throughout the country. Although it is too early to evaluate the effectiveness of the OASys, it would be helpful to reflect on the findings of the small number of research studies that have addressed the implementation of such instruments. This article draws mainly on the author's own study of the implementation of the Canadian Level of Service Inventory-Revised (LSI-R) and practice in two British probation areas prior to the introduction of the OASys. Another assessment instrument used in Great Britain prior to the development of the OASys was the ACE system (Roberts et al., 1996; Gibbs, 1999). Drawing on these study findings, the author argues that as probation areas prepare for the introduction of OASys, a level of ambivalence among staff is inevitable. Rather than being labeled as resistance, the author advises that this ambivalence should be acknowledged as a reasonable response to the increasingly technical character of probation practice. Practitioners must be persuaded that over both the short and longer term the benefits of OASys will outweigh the costs, notably the time and recordkeeping tasks involved. 24 references