This chapter examines mainstreaming duties as they exist in the European Union (EU) and the United Kingdom (UK). It also examines the commitment in EU law to mainstreaming equality and non-discrimination, as well as analysing the Public Sector Equality Duty contained in section 149 of the British Equality Act 2010 (EA 2010). The chapter explains that states that a public authority must, in the exercise of its functions, have due regard to the need to eliminate discrimination and other prohibited conduct, to advance equality of opportunity, and to foster good relations. The duty can also be referred to as a duty to mainstream equality. The duty means that public authorities need to be proactive in preventing discrimination and advancing equality. It is argued that this duty can provide a clear addition to the discussed affirmative action measures, in order to reach substantive equality or real equality in practice for all people.