This paper is a critical assessment of the theory and evidence on three general issues in comparative education. It assesses the factors affecting the origins and expansion of national educational systems; the factors influencing the organizational structure and ideologies of systems of mass schooling and higher education; and the effects of expanded educational systems for individuals, groups, and societies. It finds that the institutionalization of mass schooling is associated with nation-building processes in the eighteenth century, while universities originated in medieval Europe under papal sponsorship. In the post-World War II era, educational expansion is weakly influenced by national structural characteristics and seems to have its impetus in the rise of a transnational world culture. Participation in this wider civilizational network may explain the increasing convergence (especially at the lower levels) of educational organization and ideology. Education positively affects individuals, low status groups (women, for instance), and societal development. These effects may reflect the world-wide rise of educational credentialism. The paper concludes by advocating more explicitly comparative research that directly tests institutional-level explanations. A bibliography is included. (Author/IRT)