OBJECTIVE: This study conducted a systematic review of the systematic reviews of readability assessment studies to contribute to future readability research and practice by providing a comprehensive overview of information readability for patients. METHODS: We searched multiple databases. We included systematic reviews of studies that quantitatively assessed the readability of health information. RESULTS: This study included 24 systematic reviews, which assessed the readability of 29,424 materials across 438 studies from 1990 to 2022. All systematic reviews reported that the readability of most materials exceeded the recommended sixth to eight-grade reading level. The readability level did not improve between 2001 and 2022, when the included systematic reviews were published. CONCLUSIONS: We found that the required reading level of information was too high for patients in all clinical areas included in this systematic review. We also identified gaps in readability assessment research in clinical areas and across media types and languages, which should be addressed by future studies. PRACTICE IMPLICATIONS: Health professionals should use available guidelines to make existing patient information easy to read and to write easy-to-read patient information, thereby improving readability. Such efforts are needed regardless of the organizations these professionals work at.