This book constitutes the refereed proceedings of the 11th International Conference on the Theory and Application of Diagrams, Diagrams 2020, held in Tallinn, Estonia, in August 2020.*The 20 full papers and 16 short papers presented together with 18 posters were carefully reviewed and selected from 82 submissions. The papers are organized in the following topical sections: diagrams in mathematics, diagram design, principles, and classification, reasoning with diagrams, Euler and Venn diagrams, empirical studies and cognition, logic and diagrams, and posters.*The conference was held virtually due to the COVID-19 pandemic.The chapters 'Modality and Uncertainty in Data Visualization: A Corpus Approach to the Use of Connecting Lines, ' 'On Effects of Changing Multi-Attribute Table Design on Decision Making: An Eye Tracking Study, ' 'Truth Graph: A Novel Method for Minimizing Boolean Algebra Expressions by Using Graphs, ' 'The DNA Framework of Visualization' and 'Visualizing Curricula' are available open access under a Creative Commons Attribution 4.0 International License via link.springer.com.