The 6th International Conference on the Theory and Application of Diagrams - Diagrams 2010 - was held in Portland, USA in August 2010. Diagrams is an international and interdisciplinary conference series, which continues to present the very best work in all aspects of research on the theory and application of diagrams. Some key questions that researchers are tackling concern gaining an insight into how diagrams are used, how they are rep- sented, which types are available and when it is appropriate to use them. The use of diagrammatic notations is studied for a variety of purposes including...