This book reproduces a working document elaborated in the summer of 1999, within the framework of the Atlantic Treaty Association (ATA) by its Secretary General, Mr. Alfred Cahen, on the occasion of NATO's 50th anniversary commemorations. It provides a comprehensive insight into the changes that came about at the heart of the Alliance after the fall of the Soviet Union, and the principal questions and political problems being examined today on both sides of the Atlantic. NATO's enlargement, the deepening of a new European and Mediterranean dialogue on security and defence issues, arms control, new allied missions and the political, strategic and legal instruments created for its use, as well as its evolution, are themes also dealt with in this book. It will provide those who are interested in these questions and the public in general with an exhaustive yet succinct vision of the problems which, at the outset of the 21st century, are facing European security and defence in the Atlantic framework. This publication is dedicated to Alfred Cahen who died on 19 April 2000. The text of his report is given in its original version but it has been restored to its political context by means of an introduction by Eric Remacle and Pascaline Winand (Institut d'Etudes Européennes, Université Libre de Bruxelles).