This is the first in a series of four inter-related reports titled Self-Regulation and Toxic Stress. This report, Foundations for Understanding Self-Regulation from an Applied Developmental Perspective, provides a comprehensive framework for understanding self-regulation in context, using a theoretical model that reflects the influence of biology, caregiving, and the environment on the development of self-regulation. The second report, A Review of Ecological, Biological, and Developmental Studies of Self-Regulation and Stress, provides a cross-disciplinary review on research of the relationship between stress and self-regulation. The third report, A Comprehensive Review of Self-Regulation Interventions from Birth through Young Adulthood, describes the strength of evidence for interventions to promote self-regulation for universal and targeted populations across development. The fourth and final report, Implications for Programs and Practice, considers implications of findings from the prior reports for programs supported by the Administration for Children and Families (ACF). In the present report, the authors introduce and describe a set of seven key principles that summarize our understanding of self-regulation development in context.