International audience ; We report on the results of the Improving Classroom Assessment (ICA) project in the Netherlands that was aimed at improving primary students' mathematics achievement through improving their teachers' classroom assessment. Towards this end we first investigated primary teachers' assessment practice in a large-scale survey study. After having described and profiled teach-ers' current assessment practice, we designed a number of classroom assessment techniques, which were tested for feasibility in two small-scale sub-studies. Finally, we evaluated the effectiveness of the use of these classroom assessment techniques in a large-scale evaluation. Results indicate that students generally benefit from their teachers' improved use of classroom assessment techniques in mathematics.