The first foundational work on literacy. What is functional illiteracy? After a definition and an overview of the various aspects of literacy, theories and research approaches, societal and economic frameworks, as well as didactic perspectives are presented. The handbook provides a solid overview of the topic of literacy from both theory and practice. It serves as a foundational work that incorporates all relevant disciplines and their research findings. It is primarily aimed at students in teacher training programs and related fields in basic education. Additionally, it can be used in professional development for educators in literacy and adult basic education.