This chapter theorises ethnicity as a mode of thought and identification around which ways of being, acting and relating are organised. It is one among many possible anchors for identification, solidarity and difference, though it is the most prominent in Kenya. I discuss how this became so, describing identity and community before colonialism, and offering a history of how ethnicity organised social life under and after colonial rule, especially around elections. I provide a sketch of varied ethnic identifications in Kenya, demonstrating immense variety, not all of which obviously fit an ethnic framework, and many of which entail politics quite different from the ‘big 5’ which dominate studies of elections. Finally, I situate the case of Kenya in a comparative context, highlighting key features of how ethnic classification has operated in Kenya, including reification, colonial penetration, nationhood, demography, and differences between direct and diffuse effects of identification. This section shows that both ethnicity and its classification can be conducive to pluralism and solidarity in Kenya, but perhaps not in other contexts.