Virtually all public policy requires evidence to be enacted into law. This landmark book is the first comprehensive, multinational scoped examination of the subject which defines the concept of evidence, traces its origins in history, and shows specifically how different practices and professions differ in using evidence in their operations. The fields of science, law, and journalism are characterized in how they assess and use evidence. The book further presents a model of influence on legislators in both presidential and parliamentary governments. The twelve major influencers are identified and evaluated by the degree of influence they have over eventual policy. For practitioners, the strengths and weaknesses of numerous analytical tools are introduced. These are both quantitative and qualitative tools. Finally, evidence based policy is placed in the framework of overall management, strategy, and leadership challenges that policy leaders face.