There is no consensus on how best to assess the health literacy demands of health information materials. Comprehensive, reliable, and valid assessment tools are needed. The authors report on the development, refinement, and testing of Health Literacy INDEX, a new tool reflecting empirical evidence and best practices. INDEX is comprised of 63 indicators organized into 10 criteria: plain language, clear purpose, supporting graphics, user involvement, skill-based learning, audience appropriateness, user instruction, development details, evaluation methods, and strength of evidence. In a sample of 100 materials, intercoder agreement was high: 90% or better for 52% of indicators, and above 80% for nearly all others. Overall scores generated by INDEX were highly correlated with average ratings from 12 health literacy experts (r = 0.89, p < .0001). Additional research is warranted to examine the association between evaluation ratings generated by INDEX and individual understanding, behaviors, and improved health. Health Literacy INDEX is a comprehensive tool with evidence for reliability and validity that can be used to evaluate the health literacy demands of health information materials. Although improvement in health information materials is just one aspect of mitigating the effects of limited health literacy on health outcomes, it is an essential step toward a more health literate public.