Scientific breeding in the twentieth century greatly accelerated wheat‘s evolution, producing high-yielding varieties that helped avoid famine in many developing countries. Emerging scientific tools hold promise for identifying and tapping new, useful genetic diversity within wheat‘s primary and secondary gene pools and, through genetic engineering, beyond.The book describes generally how policies affect wheat genetic diversity; it looks at historical changes in wheat genetic diversity, as policy and priorities have evolved; it identifies factors that explain changes and differences in spatial diversity; and finally, it analyzes the productivity impacts of changes in diversity. Chapters define various types of crop genetic diversity and ways to measure them, framing the definitions and metrics in the contexts for which they are most relevant.