The theoretical foundation for the biology didactics research project "21 Theories" - each presented by recognized experts - serves as the starting point for a thorough exploration of subject-specific didactic questions. This allows anyone interested in research to quickly gain an overview even in less familiar research areas. Theories on successful teaching and learning, theories on knowledge acquisition, evaluation, and theories on the emotional aspects of learning and action are presented. The aim is to clarify the following components of a theory: to demonstrate what needs to be done to achieve a specific goal, to suggest which factors need to be controlled when planning an investigation, to anticipate the consequences of a particular action, and to enable retrospective evaluation of results.