The private sector's role in education has been the subject of much analysis and policy debate in recent years. In developing countries, public resources for education are limited and governments have traditionnally relied on private education, particularly at the post-basic levels, to meet the excess demand for education. Even when excess demand is not a major issue, advocates of private education note that private schools can be more efficient than their public sector counterparts, delivering more value in terms of student achievement per investment of resources. In this article we explore Tanzania's experience with private education during the early 1990s, a period of rapid educational expansion stimulated by new governement policies put in place in the mid- to late 1980s. We compare public and private secondary schools in terms of student achievement, as well as in terms of how they are organized and run. Taking advantage of the availability of a unique set of longitudinal data, we also examine the impact of rapide private sector growth on overall sector development, student learning, competition among schools for staff resources, and changes in student enrollments at the school level.