This article reviews the literature on place diversity and the quest to use design to promote social and economic mix in human settlements. The article fits together a large literature on the subject of the interrelationship between diversity and place and explores how diversity could be enabled within the context of the city-planning profession. It argues that the linkage between city planning—defined in its traditional sense as a profession concerned with the design of cities—and place diversity is understudied. Four distinct though interrelated theoretical bases for diversity are discussed: place vitality, economic health, social equity, and sustainability. The article argues that the promotion of place diversity requires focused effort on the part of planners, and that design-based strategies are an appropriate part of that effort.