This chapter explores diversity in the forms of gender, race and ethnicity. Discussion of gender theory must acknowledge its origins in feminism. The chapter considers how theories relating to gender, race and ethnicity have been absorbed into and expanded on by planning theory. It recognizes that planning theory has brought great insight into the way diversity and 'otherness' can be created, supported or constrained by the urban environment. The chapter presents evidence of the limited influence of theoretical developments on planning practice. It considers how these theories have influenced planning in countries experiencing rapid economic development and urbanisation resulting in social change and social and economic marginalisation of certain groups. The chapter concludes by highlighting what we see as the challenges planning theory faces in the 21st century, especially in light of growing civil unrest and right-wing politics resulting from war, fiscal austerity and international migration.