The late 1980s and the early 1990s are turning points in the history of monetary unification in Europe. It is the time of the Delors report which provided the intellectual basis of the Maastricht Treaty. The latter was signed in 1991 and developed the blueprint for monetary union in Europe. At the end of the 1990s monetary union became a fact of life in the largest part of the European Union. . What have we learned since the Treaty was signed? This is the question we attempt to answer in this paper. Let us first look at the views prevailing at the time of the signing of the Treaty. We will then return to the question of what we learned since then.