This book comprises the papers presented at a conference on monetary unions held at City University, London, in May 1999, along with the comments on them by their discussants at the conference. Monetary union was a much discussed subject at that time, of course prompted by the Economic and Monetary Union (EMU) in which some members of the European Union (EU) have joined, and has remained so since then. Most of the discussion, though, has been focused on that particular union. Few authors have stood back and considered either the relevant theory, whatever that might turn out to be, or what lessons might be drawn from other unions that have been formed, some durable, some not, in the past. It is the aim of four of the papers gathered together in this volume to address these issues; and although the fifth does look directly at EMU, it does so with the aid of analytical tools not previously used on that subject.