This paper provides a synthetic overview of the complex dimensions that shape the interrelations between the massification of higher education systems and their structure and composition. Many higher education systems worldwide expanded extensively in the last decades, and have undergone wide and deep structural changes. Most notably, the diversity of many higher education systems has increased dramatically, both horizontally and vertically. We address in this paper the following issues: external and internal boundaries of higher education systems; top-down and bottom-up forces; globalization and supra-national trends; flexibility; and public and private sectors. These themes highlight most crucial issues to which higher education policy makers are advised to pay attention to when deliberating what are the best ways to implement the expansion of their higher education systems, and what should be the desirable diversity in their national contexts.