The argument of this article is that changes in curriculum need to be closely linked to changes in assessment and that this is true as much of the forms of assessment as it is of its content. Using science as the case in point, the changes in the goals of science education in the 1960s towards a greater emphasis on inquiry skills were matched some 20 years later with a change in assessment to include performance assessment. Now the new goals of science education are focused on the need to link science to the broader social context, but assessment practices have yet to catch up with this change. Given the relatively greater importance on assessment in the present era, the new curriculum emphasis may well be ignored unless new approaches to assessment are not designed and implemented soon.Developing valid and reliable assessment instruments is complex at the best of times. However, at times of major change in the curriculum, additional challenges and dilemmas present themselves to test developers and force questions about the sometimes competing roles of assessment in the larger educational context. In times gone by, such competing roles might have been of only academic interest. At the present, however, assessment-whether international, national or local-has become of such importance, both educationally and politically, that clarifying the roles and purposes of assessment has become a priority.In the past 10 years, I have had the opportunity of participating in or closely observing several science curriculum development projects and also two science assessment projects. The curriculum development projects have been in the Canadian context and speci cally in the province of Ontario, but in the course of undertaking these projects I have also had reason to analyse science curriculum developments elsewhere in the world. The assessment projects with which I have been associated have included the Third International Mathematics and Science Study (TIMSS)-a large international study for which I acted as science coordinator-and the Assessment of Science and Technology Achievement Project (ASAP)-an Ontario project to develop curriculum and assessment resources for classroom teachers. Despite the differences in purpose of these two projects, they shared the challenge of developing valid science assessment instruments in a period of signi cant curriculum change.