At the time of the development of the new curriculum in Slovenia, international comparative data providing information on the performance of Slovene students as compared to other relevant countries were available from a limited number of assessments. For this reason, it was even more important that additional international comparisons were conducted for the purposes of evaluation of the reform. In addition to several other international studies in this period, Slovenia participated in all three data collections for the Trends in Mathematics and Science Study (formerly named the Third International Mathematics and Science Study; 1995, 1999, 2003). The TIMSS data represent a rich source of information on the Slovene education system. This paper discusses the use of these data for policy making in the context of the Slovene reform and the issues of validity related to the use of these data for the purpose of evaluation of the reform. In particular, it is shown how a thorough understanding of the conceptual design, methodology and implementation of the data collection are of vital importance for making useful interpretations of the results. Furthermore, this paper presents a short overview of how international assessments can be used for purposes of national monitoring of education and how this applies in the context of the Slovene education system. It also illustrates and discusses the utilization of the TIMSS data in Slovenia. (Contains 1 figure, 2 tables, and 1 note.)