Robert Neuwirth challenges conventional thinking by examining the world's informal economy close up. To do so, he spent four years living and working with street vendors and gray marketers, to capture its scope, its vigor--and its lessons. He calls it “System D” and argues that it is not a hidden economy, but a very visible, growing, effective one, fostering entrepreneurship and representing 1.8 billion jobs worldwide.  It's an economy of underappreciated power and scope. Our challenge, Neuwirth says, isn't to end squatter cities or shut down gray markets--but to engage and empower those who live and work in them.