Every four years since 2004, the Copenhagen Consensus Center has organized and hosted a high profile thought experiment about how a hypothetical extra $75 billion of development assistance money might best be spent to solve twelve of the major crises facing the world today. Collated in this specially commissioned book, a group of more than 50 experts make their cases for investment, discussing how to combat problems ranging from armed conflicts, corruption and trade barriers, to natural disasters, hunger, education and climate change. For each case, 'Alternative Perspectives' are also included to provide a critique and make other suggestions for investment. In addition, a panel of senior economists, including four Nobel Laureates, rank the attractiveness of each policy proposal in terms of its anticipated cost-benefit ratio. This thought-provoking book opens up debate, encouraging readers to come up with their own rankings and decide which solutions are smarter than others.