This dissertation explores the effects of European integration on national level legislatures in European Union member states. Many economic and political decisions previously made by democratically elected national legislatures are now decided at a supranational, or European, level. National legislatures have responded to this change by increasing their oversight of European affairs in order to impose more democratic accountability and transparency into the opaque decision-making process of developing European legislation. While all national legislatures of European Union member states have established European Affairs Committees to help strengthen oversight of European affairs, there is considerable variation in the institutional strength and political control invested in these committees. I argue that the more recently a country joined the European Union, the more quickly the legislature will move to establish an oversight committee for European affairs and the more assertive its European Affairs Committee is likely to be. Cross-sectional statistical analysis shows a strong correlation between the timing of membership and the urgency with which the European Affairs Committee is established. There is also a strong correlation between timing of membership and a legislature's approach to parliamentary oversight of European affairs. Case studies on the German Bundestag, the Irish Oireachtas, and the German Bundesrat explore three different approaches to the oversight of European affairs.