The informal economy represents a significant share of output and employment in many developing countries. Yet little is known about this hidden engine of innovation. This pioneering study addresses some crucial questions, including: what is the role of the informal sector in economic development? How does innovation occur in the informal economy? How does it spread, who are the key actors and what impacts does it have? How do inventors and entrepreneurs in the informal economy reap benefits from their innovations? What stops informal sector innovation from scaling up? How can informal sector innovation in developing countries be measured? And what policies might support informal sector innovation and improve its impacts? This book will stimulate further work on this crucial but under-researched subject. As well as rich empirical evidence from several groundbreaking studies, it includes conceptual and methodological tools and policy recommendations to help researchers and policy-makers understand innovation in the informal economy