Homeland Security continues to struggle to define itself as a field of practice and scholarship. The difficulty in defining the field has led to a variety of conflicts over membership, content, and focus. This article reviews some of the prominent debates over the meaning of homeland security as a field of study and practice. It then defines a simple schema for definitions of homeland security inspired by the academic and legislative debates over the issue. A frequency cataloging of definitions from US state agencies illustrates the continued relevance of a “partial membership” approach to defining the field. The article concludes with a discussion of the implications of the diversity of homeland security definitions for the development of the field.