This chapter explores horizontal diversity in higher education in the context of high participation systems (HPS), focusing on differences in institutional mission, form and type, and internal diversity within institutions. The chapter starts with an analysis of scholarly approaches to diversity. The dominance of the market diversity perspective (‘deregulate to create more choices’) indicates not its profound relevance to the diversity issue, but the tenacious hold of marketization narratives on the policy imagination. Competition in higher education is mostly associated with less, not more, diversity. The chapter discusses four propositions in relation to diversity in HPS, highlighting decline in the overall diversity of institutional form and mission despite growth in systems, the rise of large multi-purpose institutions as the dominant form, and increased internal institutional diversity. When HPS are rendered more competitive in government-fostered quasi-markets, horizontal distinctions of mission tend to become vertical.