Sustainability continues to be a serious concern for Primary Health Care (PHC) interventions targeting the death of millions of children in developing countries each year. Our work with over 30 Non-Governmental Organizations (NGOs) implementing USAID's Child Survival and Health Grants Program (CSHGP)-funded projects revealed the need for a study to develop a framework for sustainability assessment in these projects. We surveyed NGO informants and project managers through semi-structured interviews and questionnaires. This paper summarizes our study findings. The NGOs share key values about sustainability, but are skeptical about approaches perceived as disconnected from field reality. In their experience, sustainable achievements occur through the interaction of capable local stakeholders and communities. This depends strongly on enabling conditions, which NGO projects should advance. Sustainability assessment is multidimensional, value-based and embeds health within a larger sustainable development perspective. It reduces, but does not eliminate, the unpredictability of long-term outcomes. It should start with the consideration of the 'local systems' which need to develop a common purpose. Our ability to address the complexity inherent to sustainability thinking rests with the validity of the models used to design interventions. A participant, qualitative research approach helped us make sense of sustainability in NGO field practice.