The relation between liberty and security has been highly contestable over the past 10 years in the EU integration process. With the expansion of the EU’s powers into domains falling within the scope of the Area of Freedom, Security and Justice, liberty and its relation to security has brought a new range of issues, struggles and debates. Acts of political violence labelled as ‘terrorism’ and human mobility at the European and international levels have justified the construction of these phenomena as threats to the security and safety of the nation state. They have legitimised the development of normative responses that go beyond traditional configurations and raise fundamental dilemmas for the security and liberty of the individual. This paper assesses the ways in which the notions and perceptions of security and insecurity in the EU have evolved as political values and legal/policy goals, and how they are being transformed. It aims at synthesising the results of the research conducted since 2004 by the Justice and Home Affairs Section of CEPS through the CHALLENGE project (Changing Landscape of European Liberty and Security). The research has been premised upon one basic, but determining question: To what extent has the evolution of the international context altered the dynamics of liberty and security in the EU?