This report is a product of a network to network collaboration among groups involved with the Canadian Partnership for Children’s Health and Environment and the Ontario Chronic Disease Prevention Alliance (CPCHE and OCDPA) to provide a baseline for further collaboration. The central focus is a scoping review of evidence for associations between early life environmental exposures and the later development of several of the most common chronic diseases. Part One provides context about chronic disease and care in Canada’s aging population, the reality of the multiple determinants of health, the primacy of the social determinants of health, the considerable breadth of environmental influences on health, the importance or early child development to lifelong health and the scope and complexity of multiple inter-relationships among all these health determinants. Part Two introduces the Developmental Origins of Health and Disease (DOHaD) concept and the related and expanding field of epigenetics. It also discusses key issues that arise in evaluating evidence in complex environmental health issues.