Definitions, by their very nature, establish a shared understanding of words and concepts but also set parameters for inquiry and measures. Health literacy, a term that emerged in the 1990s, has been defined in numerous ways over time and is still considered an evolving concept. This chapter provides a discussion of the difficulties inherent in restricted definitions that have led to research gaps. The discussion highlights the need for an expanded understanding of health literacy and it identifies missing elements. A call for new measures includes attention to a full range of literacy skills including calibrations of health professionals' communication skills. In addition, it argues for an in-depth understanding of health-related tasks and texts that will yield insights for a more thorough analysis of links between and among literacy skills, health system demands, and health outcomes. Finally, this chapter presents an argument for a careful consideration of institutional and system wide norms, policies, and regulations that facilitate or impede access to health information, services, and care. As the definition of health literacy expands so too can the scope and depth of health literacy research, practice implementation, and public policy.