Robert Neuwirth, author of "Shadow Cities," finds the world's squatter sites -- where a billion people now make their homes -- to be thriving centers of ingenuity and innovation. He takes us on a tour.

Robert Neuwirth spent four years among the chaotic stalls of street markets, talking to pushcart hawkers and gray marketers, to study the remarkable "System D," the world's unlicensed economic network. Responsible for some 1.8 billion jobs, it's an economy of underappreciated power and scope.

From rides to homes and beyond, we're sharing everything these days, with the help of digital tools. But as modern and high-tech as the sharing economy seems, it's been alive in Africa for centuries, according to author Robert Neuwirth. He shares fascinating examples -- like apprenticeships that work like locally generated venture capital and sy...

Designer Alastair Parvin presents a simple but provocative idea: what if, instead of architects creating buildings for those who can afford to commission them, regular citizens could design and build their own houses? The concept is at the heart of WikiHouse, an open source construction kit that means just about anyone can build a house, anywhere.