This thesis builds on the established body of research into the suitability of a country joining other countries in a monetary union by focusing on the potential costs resulting from the loss of monetary policy independence that is a corollary to forming a monetary union. We continue in the tradition of several other authors by extracting supply and demand shocks for a range of countries from VAR analysis and comparing the symmetry of these economic shocks between potential members of a monetary union. The theoretical contribution of this thesis is that we explicitly incorporate the size of each potential currency union member in the analysis. This contribution is motivated by the observation that a large country would be a more significant part of a given currency union than would a small country. Thus the monetary policy settings of a given currency union would to a larger extent reflect the economic dynamics of a given country the larger that country is relative to the size of the union overall. Previous authors have largely neglected this issue. We explicitly incorporate the size of each potential members' economy in our analytical framework and re-assess the merits of a range of regions forming a currency union. Using the framework developed, we also inquire into the optimality of current monetary regimes in two regions, the North American continent and in Australia. The first of these is motivated by Mundell's seminal article on currency unions where he asked in largely qualitative terms whether the US and Canada are better currency realms than a hypothetical north south divide of the continent. The second is motivated by the observation that Australia’s economy embodies (economically) very different sub-regions due to the difference importance of commodities production in different parts of the country. We ask whether these different regions experience symmetrical or largely idiosyncratic shocks and find support for the latter.