Diagrammatology is a scholarly book that explores the role of diagrams in thinking and knowledge. It is based on the general theory of diagrams found in the mature works of Charles Peirce and argues that diagrams represent a central element of epistemology. The book reflects Peirce's views in the context of Husserl's contemporary teaching of 'categorical intuition' and highlights the often-overlooked similarities between Peircean semiotics and early Husserlian phenomenology. Diagrams enable the observation and experimentation of ideal structures and objects, opening access to the synthetic a priori of the regional and formal ontology of the Husserlian tradition. The second part of the book focuses on three regional branches of semiotics: biosemiotics, image analysis, and literary theory. These areas are made accessible through a diagrammatological approach that transcends the traditional relativism and culturalism of semiotics, thereby establishing a realistic semiotics.