This chapter introduces the supposed problem of ethnicity: that it undermines national cohesion, or is a colonial hangover with no appropriate place in political life. In contrast, I argue that ethnicity is neither inherently desirable nor undesirable; its political effects depend on how it is known and used, and our understanding of how it is known remains underdeveloped. I establish that there is no definitive list of Kenya’s ethnic groups, and we must stop taking for granted what we think we know about ethnicity. I offer the concept of cultivated vagueness – a widespread aversion to resolving the ambiguity of lists of Kenya’s ethnic groups – to understand how ethnic knowledge works and to contrast it with legibility and governmentality. Cultivated vagueness is the response from bureaucrats, civil society, citizens and the state to the conundrum that ethnic knowledge is both common sense and impossible to settle. It also explains how ethnic classifications serve both projects of division and of pluralism. I suggest that attention to the benefits of cultivated vagueness may facilitate the advancement of the latter over the former. The chapter outlines the book’s methodology and chapters.