This thesis sets out to consider the constitutional implications of a policy of legal integration regarding internal security matters in the EU. It argues that constitutional theory is fundamental to addressing the legality and accountability concerns raised about the developing legal practice of the EU’s ‘Area of Freedom, Security and Justice’ (AFSJ). Conducting such a study poses important questions about how to pursue a constitutional approach to legal and political practices which do not resemble in any straight forward way the constitutional tradition of the nation state. This thesis advances the argument that constitutional theory cannot properly be construed as a ‘tool-kit’, a set of rules or principles with universal validity to cause a state of affairs or event then dubbed as legality or accountability. Instead it is argued that constitutional theory must work to reveal the issues of restraint, accountability or legality that are in fact part of an ongoing practice, not a one-off settlement; in which the theorist attempts to disclose or reveal the meaningfulness of what is described as the ‘common experience of the political way of life’. Consequently the first part of the thesis outlines how constitutional theory can establish the features of the ‘common experience of the political way of life’ (also referred to as the ‘constitutive function’) and it explains that meaning is situated in a historical background, which is uncovered by the theorist by providing an interpretation of this background. The thesis therefore advances and defends an interpretive theory of legal scholarship. These methodological parameters provide an appropriate means of making sense of the developments in the EU concerning the AFSJ, which thereafter becomes the focus of the thesis. In particular it concentrates on the importance of developing an understanding of public goods that form the basis as to why it is possible to think in constitutional terms about the AFSJ. The approach taken to public goods is that they manifest the meaningful commitments of a political community and therefore cannot be construed in instrumental terms. The thesis outlines that the constitutional issues facing the AFSJ are often collapsed into matters of instrumentalism that conceals the need to engage with the on-going meaning of the practices as forming part of a common political way of life. It is argued in conclusion that the thesis has provided a more robust way of not only considering the challenges facing the emerging internal security policies of the EU but has also provided an appropriate theoretical approach for the study of such issues in constitutional theory.