Book excerpt: “The fourth sector” is a relatively new sector that consists of for-benefit organizations that combine market-based approaches of the private sector with the social and environmental aims of the public and non-profit sectors. This book examines trends of entrepreneurship in the fourth sector, describes specific ecosystems fostering new ventures around the world, and characterizes the most common and innovative business models. It covers as well the main effects, among others, of technological change, innovation, and institutional behavior on the sector in the last years.