The European Union has entered a period of progressive dynamism since the late 1980s, as shown not only by its transformation into a larger European Union with an ambitious policy agenda as envisioned by the Treaty on European Union (TEU) but also by an oncoming wave of applicants for membership. 1 Although various political scientists have argued that integration theory is still relevant, 2 it has generally not been modernized to take into account the changes to the Union's size and structure or the changes in the views of various applicants. The purpose of this chapter is to assess the implications of successive and future enlargements on European integration theory and vice versa. 3 The following section provides a short survey of integration theory. After a brief examination of economic theories, it focuses on political theories, particularly intergovernmentalism and neofunctionalism, and modifies the concept of" spillover." The third and fourth sections evaluate the relevance of these theories to past (1970s and 1980s), current, and future (1990s) enlargements respectively, arguing that enlargement has important implications for integration theory and that a revision of integration theory needs to be undertaken. This is pursued at length in the penultimate section, which highlights the drawbacks of neofunctionalism and suggests that intergovernmentalism provides a more appropriate paradigm for the examination of enlargement. However, it does not discard neofunctionalism, but, rather, it revises the concept of" spillover" to maintain its relevance to European integration within a predominantly intergovernmental framework. 4