On November 15, the world population officially surpassed 8 billion people, according to United Nations estimates. The milestone was reached just 12 years after the world marked 7 billion people, and it is projected that global population will hit 9 billion people by 2037 So what are the key demographic trends driving population growth? Where is population growing fastest? Where is it declining? And what do the age structures of populations around the world tell us about the future of our planet? To answer these questions, we are joined by John Wilmoth, head of the Population Division at the United Nations Department of Economic and Social affairs.  We kick off with a brief history of population growth and with a discussion of the so-called demographic transition, which is the long process in which populations steadily live longer and have few children.