The concept of theme is critical to the accurate interpretation of qualitative data. A literature review of qualitative research methodology and nursing research studies reveals considerable diversity in the identification of themes, the interpretation of the concept, and its function in data analysis. Part of the problem is the transfer of research methods from other disciplines to the study of nursing phenomena. The transfer often results in a blending of distinct research methods that compromises methodological rigor in data analysis and theory generation. A definition of theme is developed from the literature review and interdisciplinary definitions and descriptions. Five aspects of a theme and criteria foundational to the definition and concept of theme are identified. Implications of the concept of theme are presented for the conduct and application of research findings to the practice and development of nursing science.