The purpose of this article is to outline a framework of explanation of the unique tradition of comprehensive schooling in Scandinavia. All the countries developed an all-through system of education from grade one to nine/ten with mixed ability classes for nearly all. This all-through system of education is a product of a long historical development. It will be argued that four factors shaped this development: strong state involvement, a relative egalitarian class structure, powerful Liberal Party and a strong Social Democracy.