"Official relations between the European Union (EU) and the North Atlantic Treaty Organization (NATO) newly began after the establishment of the European Security and Defence Policy (ESDP) in 1999. Before that we can not talk about formal relations between the two organizations". So in this Book, we will describe the historical development of EU-NATO relations from post-Cold War (1990) until the Berlin Plus agreement in 2002, and will analyze this agreement which is considered as the core of the relationship between the two organizations. In addition, we will examine the military missions which were carried out under Berlin Plus agreemnt (Operation Concordia, which took place in the former Yugoslavs Republic of Macedonia in March 2003) and (Operation Althea, which took place in Bosnia and Herzegovina in 2004), in order to understand if the Berlin Plus agreement applied or not, and to know whether there was real cooperation between them or not.