His main area of expertise is constitutional theory. He has published extensively on the constitutional dimension of legal order at sub-state, state, supranational and international levels. He has also published at length on the relationship between security, legal order and political community. He maintains a more general interest in broader questions of legal theory as well as in various substantive dimensions of UK and EU public law.

Previously he taught public law at Edinburgh for ten years (1986-96), was Professor of Legal and Constitutional Theory at the University of Aberdeen (1996-2000), and, most recently, was Professor of European Law at the European University Institute in Florence (2000-8), where he was also the first Dean of Studies (2002-5).

He has also held various visiting appointments - including Visiting Professor, Department of Philosophy, University of Tilburg, Netherlands (2000); Visiting Professor of Law, University of Columbia, NY(2005); Eugene Einaudi Chair of European Studies, University of Cornell (2007); and Distinguished Visiting Professor of Law, University of Toronto (2007).

Professor Walker's inaugural lecture, entitled "Out of Place and Out of Time: Law's Fading Co-ordinates", was delivered on the 18th November 2008. You can listen to it from the Blogs and Podcasts section of the School website.

In December 2008 Professor Walker was asked by the Scottish Government to conduct an independent review of final appellate jurisdiction in the Scottish legal system. His final Report, submitted in January 2010, can be found at:

 http://www.scotland.gov.uk/Publications/2010/01/19154813/0

This book develops and defends an original argument concerning the importance and value of security as a good, and the virtue and necessity of the democratic state in fostering and sustaining that good. It travels widely over debates in social and political theory, sociology, criminology, law and international security studies.

International co-operation and criminal law enforcement have become a centrally important policy issue for Europe in the 1990s. This book examines all of the major empirical and theoretical issues associated with the emerging pattern of co-operation, including the harmonization of criminal law and criminal procedures, law enforcement strategies, police organization and discipline, and the politics of immigration and civil liberties.

This book sets out to examine some of the key features of what we describe as the paradox of constitutionalism: whether those who have the authority to make a constitution - the 'constituent power' - can do so without effectively surrendering that authority to the institutional sites of power 'constituted' by the constitutional form they enact. In particular, is the constituent power exhausted in the single constitutive act or does it retain a presence, acting as critical check on the constitutional operating system and/or an alternative source of authority to be invoked in moments of crisis? These questions have been debated both in different national contexts and at the level of constitutional theory, and these debates are acknowledged and developed in the first two sections of the book.

Part I includes chapters on how the question of constituent power has been treated in the constitutional histories of USA, France, UK and Germany, while Part II examines at the question of constituent power from the perspective of both liberal and non-liberal theories of the state and legal order. The essays in Part III consider the operation of constitutionalism with respect to a series of contemporary challenges to the state, including those from popular movements below the level of the state and challenges from the supranational and international levels, and they analyse how the puzzles associated with the question of constituent power are played out in these increasingly important settings

This volume brings together a collection of classic and contemporary texts which engage with the core problem of sovereignty from the perspective of various legal and law-related sub-disciplines: legal history and theory, constitutional law, international law and relations and EU law. Many of the highlights from the intense debates about the continuing relevance or otherwise of the internal sovereignty of national legal orders and the external sovereignty of states in a rapidly- globalizing world are reproduced here.

This collection brings together leading specialists in the areas of European Union law which are now organized under the Area of Freedom, Security and Justice (AFSJ). The concept of the AFSJ was introduced into the EU Treaty framework by the Treaty of Amsterdam in 1997, and it incorporates migration law, family reunion law, asylum law, police cooperation, and cooperation in criminal law. Each of these areas of law is the subject of an in-depth examination in a separate chapter of this book.
The early years of the AFSJ, building upon a substantial body of law already in place under the Treaty of Maastricht and various intergovernmental arrangements, have witnessed a rapid expansion in legislative and executive activity in the field of European internal security. In migration law, family reunion law, asylum law, police co-operation, and co-operation in criminal law, the scale and intensity of action at the supranational level is already such as to overturn longstanding assumptions about the priority of national law in matters of migration control and criminal justice.

An introductory chapter examines the various policy strands covered by the AFSJ; investigates what, if anything, can be viewed as its distinctive legal underpinning; and discusses its possible future development in the light of current discussions over the adoption of a first documentary Constitution for the European Union. In addition to setting out the main contours of legal policy, each chapter examines the continuing tension between national sovereignty on the one hand and a growing commitment to collective, EU-wide action on the other. The volume also addresses the wider constitutional implications of a growing supranational capacity in questions of the priority of political values in the evolving EU; fundamental rights protection; the control of new forms of executive and administrative discretion; and the pressures of accommodating the ten new Enlargement states within the internal security field.

Constitutional discourse has perhaps never been more popular, nor more comprehensively challenged than it is today. The development of new constitutional settlements and languages at state and post-state level has to be balanced against the deepening of a formidable range of sceptical attitudes. These include the claim that constitutionalism remains too state-centered, overstates its capacity to shape political community, exhibits an inherent normative bias against social developments associated with the politics of difference, provides a language easily susceptible to ideological manipulation and, that, consequent upon these challenges, it increasingly represents a fractured and debased conceptual currency. A rehabilitated language of constitutionalism would meet these challenges through a version of constitutional pluralism. Constitutional pluralism recognises that in the post-Westphalian world there exists a range of different constitutional sites and processes configured in a heterarchical rather than a hierarchical pattern, and seeks to develop a number of empirical indices and normative criteria which allow us to understand this emerging configuration and assess the legitimacy of its development.

(Note: Italian version “Una costituzione con la “C” maiuscola o con la “c” minuscola?” published in (2005) 25 Quaderni costituzionali 881-884.)

This paper considers whether, why and to what extent we should conceive of transnational regulation in constitutional terms. It distinguishes between two different candidates for transnational constitutional status. On the one hand, there are various actual or potential 'holistic' transnational constitutions, such as the EU and the WTO. These constitutional orders resembe that of the state to the extent that they involve the framing of a distinct 'body politic'. This 'body politic' may be thinner or thicker, depending on the number and richness of the framing layers involved (legal order, politico-institutional complex, popular self-authorization, distinct society or demos), but the idea of the constitution as a constituent dimension and expression of a broadly encompassing, internally coherent and externally bounded polity is present in all cases. On the other hand, there are also today various international societal actors and functional spheres (e.g internet regulation, sport regulation) that on one view possess their own 'societal constitutions'. Unlike holistic forms of regulation, however, these areas tend to combine very narrow forms of self-regulation with diverse forms of external regulation. The idea of a discrete framing is not present even in legal or institutional terms, still less in popular or social terms. Nevertheless, the paper argues, there may be good normative reasons for continuing to use the language and mindset of constitutionalism in these contexts.

Modern jurisprudence has been dominated by questions of authority and questions of meaning. The present paper addresses what it calls the third question of jurisprudence - namely the question of how law situates itself in space and time. This is a question to be addressed at various levels, the most important of which concerns the overall legitimacy of law as a normative enterprise which claims to come from somewhere and 'somewhen' and to be directed to somewhere and 'somewhen'. In the Westphalian tradition in which the dominant forms of law have been constitutional law and international law, this question of self-situation has tended to be answered either in highly universalistic terms (a law that transcended place and time) or in highly particularistic terms (a law that was peculiar to a particular place and perpetual in its ambition) . Both universalistic and particularistic narratives, despite their superficial contrast, emphasize the holistic and magisterial properties of law - as a form of authority that is both comprehensive and self-contained and sovereign within its situation. Today, with the fading of the Westphalian paradigm and the growth of myriad new forms of transnational law, various forms of new regulation - pluri-constitutive, interstitial, non-constitutive and global general law - lack the background co-ordinates either of universalism or of a comprehensive particularism and the associated systemic qualities of holism and magisterialism. What do these new forms of 'uncharted law' suggest more generally about the future of the legal form and of the ways in which law may legitimate itself?

The category of the denizen is becoming increasingly important in the identity politics of the EU. EU law and policy over a number of years has encouraged the development of a new hybrid status of the permanent resident who possesses many legal and social rights but lacks full political citizenship. Thinkers and politicians differ over the implications of this development, some seeing it as a temporary status on the way to full citizenship, others seeing it as a permanent sub-citizenship status, and others still seeing it as a way of moving beyond the citizenship/non-citizenship dichotomy in understanding the relationship between individuals and political communities. The paper explores this third alternative at some length, and concludes that the figure of the denizen may indeed be an appropriate archetype for imagining political community at the supranational level.