The Council on Foreign Relations created an impressive microsite on the issue of child brides and underage marriage around the world. In all, 142 million girls will marry in the next decade before the age of 18.
The worst offenders are India, Bangladesh and Nigeria. This map (interactive version here) shows where the problem is gravest.
Do have a look at the microsite, which contains useful information about the scale of the problem and makes a compelling argument for why stopping child marriage should be a priority of international relations.