Throughout the past several decades, the boundaries between the public, private, and social sectors have become less defined as there has been a significant emergence of practices and new organizations that are blending social and environmental missions with business practices. This paper examines the forces at work causing this emergence and the implications it has for each individual sector and society as a whole. Furthermore, it makes the case that new organizations sprouting from the blurring of sectoral boundaries constitute what is referred to as a “Fourth Sector” – organizations that are driven by a social mission and employ business policies and practices. It also suggests a pathway for accelerating the development of this new sector through the creation of a new class of “For-Benefit” organizations and the formation of a supportive ecosystem that is optimized to their needs. Written by Heerad Sabeti, co-founder of the Fourth Sector Network, this paper is supported by the Aspen Institute’s Program on Social Innovation and the W. K. Kellogg Foundation. The executive summary is currently available as a free download.