In the field of school effectiveness and school improvement, scholars as well as practitioners often complain about the absence of theory to guide their work. To fill this gap, Creemers (1994) developed a comprehensive model of educational effectiveness. In order to gain empirical evidence, we tested some of the main components of the model in lessons of mathematics in the lst year of lower general education in The Netherlands. The results show that the main factors in the model-time spent, opportunity to learn, and the quality of instruction-are most important in predicting achievement. No evidence was found for the relationships between levels.