This chapter focuses on the member states of the European Union. It first considers six factors that determine how a state engages with the EU: the date of entry, size, wealth, state structure, economic ideology, and integration preference. It then examines how member states behave in the EU’s institutions and seek to influence the outcome of negotiations in Brussels. It also discusses the informal and formal activities of the member states before concluding with an overview of the insights offered by theory in analysing the relationship between the EU and its member states. The chapter clarifies some key concepts and terms such as Europeanization, acquis communautaire, and flexible integration, and explains how the EU’s intergovernmental conferences work.