The United States can be thought of as a non-trivial currency union since, although it is a single country, it encompasses many states that have economies comparable in size to those of some European countries. Given that the United States has had a single currency for 200 years, it may be thought of as a successful currency union. Yet many of the US states produce very different products and services, so that they are likely to be impacted by different kinds of macroeconomic shocks (expansionary and recessionary) over time. For example, Texas produces oil, while Kansas produces agricultural goods. How do you explain the long-term success of the US currency union given this diversity? Are there any lessons or predictions for Europe that can be drawn from the US experience?