The CORE Group Polio Project (CGPP) has contributed to polio eradication by successfully engaging civil society, particularly the non-governmental organization (NGO) community. This engagement, which began with a grant from the U.S. Agency for International Development in 1999, has contributed to improvements in routine immunization programs, polio campaign quality, and surveillance for acute flaccid paralysis in many challenging geographic areas. The CGPP has worked closely with polio eradication partners in a collaborative and supportive role. The CGPP has focused largely on high-risk areas with marginalized or hard-to-reach populations where health systems and immunization programs have also been weak and where transmission of poliovirus had not been stopped. The CGPP has engaged local civic leaders and communities in ways to complement top-down vertical efforts of ministries of health and other partners in the Global Polio Eradication Initiative. The CGPP has developed innovative strategies to detect cases using community-based surveillance, promoted independent campaign monitoring, established cross-border initiatives, and developed a strong and creative cadre of community mobilizers to track missed children and deliver behavior change education. Many of the innovations and approaches that the CGPP helped to develop are now being replicated by governments and international agencies to tackle other public health priorities in underserved and marginalized communities around the world. This article is the first in a series of articles describing the work of the CGPP. Because the article describes the work of more than 40 NGOs in 11 countries over 20 years, it provides only an overview, leaving many important details and variations of the CGPP's work to be described elsewhere, including in other articles included in this series.