In some primary schools, the average performance of pupils over several years is significantly below the level that could be expected of these pupils. There are several theories for this phenomenon, known as underperformance. Theory on opportunity to learn predicts that pupils in underperforming schools are not given sufficient opportunity to attain the minimum objectives of the curriculum. Contingency theory predicts that activities of principals, teachers, and school boards mediate insufficiently between the educational process and situational factors. The compensation hypothesis predicts that schools in disadvantaged areas first have to compensate for the fact that their pupils lag behind and provide for their basic needs before they can work on structural improvement of educational processes. The additivity hypothesis predicts that schools in disadvantaged areas have a higher risk of low output, even after correcting for social and economic background. Evidence can be found in support of each of these theories and hypotheses. (Contains 2 figures and 3 tables.)