In 2014 three new procurement directives were adopted at European level, replacing the previous generation of directives from 2004. These directives regulate how approximately €2 trillion of public and semi-public money is spent in the member states, aiming to ensure the free movement of goods and services and competition in the award of public contracts. Environmental and social provisions figure prominently in the 2014 directives, including a number of new rules which must be implemented at national level. The 2014 directives embody the concept of a social market economy as set out in the Lisbon Treaty, and represent an increase in EU integration in the public procurement field. This thesis analyses the role of the European Commission, Council, Parliament and Court of Justice during the reform process, as well as the policy preferences of France, Germany, the UK and civil society groups. It asks whether the EU institutions acted as supranational policy entrepreneurs or as agents of the member states in introducing the new social and environmental provisions, testing hypotheses derived from two competing theories of integration. It draws conclusions about the nature and causes of European integration in this field, and develops the concept of trusteeship in relation to the Court of Justice and European Parliament as a means of understanding democratic governance in the EU.