National tests, an influential assessment practice in many countries in East Asia, are often blamed as obstacles to good instructional practices. In this paper, the positive impact of assessment, national tests in particular, is discussed using the case of Singapore. The first part of the paper includes an analysis of items from the primary grade national test. It is found that there is a significant emphasis on visualization, number sense, communication and metacognition, and an emerging emphasis on patterning. The impact of allowing students to use calculators in the national test is also discussed. The second part of the paper focuses on the impact of this emphasis on classroom instruction, textbook materials and professional development of teachers. This paper aims to show that, instead of being an obstacle to good instructional practices that develop mathematical thinking, large-scale assessment such as national tests can be the catalyst. (Contains 2 tables and 6 figures.)