National systems of higher education became more diversified in the process of expansion. They vary substantially, however, according to the extent of diversity. Also, the major dimensions of diversification tend to play different roles: types of institutions, types of programmes, levels of programmes and degrees, and variations in reputation and prestige within formally equal institutions and programmes. Theories provide various explanations regarding the dynamics of diversification and the role the different dimensions play. Actually, some countries reduced the role of diversification according to institutional types in the 1990s, while others established new types. The "Bologna Process" underscores a growing role of levels of programmes and degrees in most European countries. It remains to be seen what impact these changes have on the stratification of the higher education systems and with respect to the encouragement or discouragement of individual institutions to develop specific profiles and thus to contribute to horizontal diversity.