One priority in nursing research is the development of middle-range theories that are applicable to nursing practice. The purpose of this article is to describe the development of a middle-range theory, adapting to diabetes mellitus, through theory and concept synthesis. The adaptation to chronic illness model explicates the conceptual relationships of the Roy adaptation model in a theory specific to chronic illness. Several concepts in the diabetes literature, self-management, integration, and health-within-illness were synthesized with the theory through examination of empirical evidence. This method of theory development increases the specificity of the existing theory and informs the application to nursing practice.