This chapter provides motivation and an overview of the main issues discussed in the book. The Industrial Revolution unleashed an era of economic and population growth, fueled by innovation, capital accumulation and international trade. This process opened unlimited opportunities for the progress of Humanity. However, the environmental consequences of this progress are already testing the capacity of the planet to absorb its impacts. In the most recent decades, the trade off between economic progress and sustainability has become evident, while there is still a large fraction of humanity that lives in unacceptable poverty, or aspires to achieve consumption levels like those of more advanced countries. To accommodate both goals, major changes must take place: population growth must slow down and eventually become negative, as is already happening in most of the world, except in the poorest areas. Consumption of physical goods and energy must decline in more advanced nations to leave room for expansion in low-income countries and the environmental impact of human activities per unit of output must decline significantly. Development economics must bring back Nature into the design and application of Economic and Social policies.