Book excerpt: The world has many pressing problems. Thanks to the efforts of governments, NGOs, and individual activists there is no shortage of ideas for resolving them. However, even if all governments were willing to spend more money on solving the problems, we cannot do it all at once. We have to prioritize; and in order to do this we need a better sense of the costs and benefits of each 'solution'. This book offers a rigorous overview of twenty-three of the world's biggest problems relating to the environment, governance, economics, and health and population. Leading economists provide a short survey of the analysis and sketch out policy solutions for which they provide cost-benefit ratios. A unique feature is the provision of freely downloadable software which allows readers to make their own cost-benefit calculations for spending money to make the world a better place.