This book constitutes the refereed proceedings of the 6th International Conference on Theory and Application of Diagrams, Diagrams 2010, held in Portland, OR, USA, , in August 2010. The 22 revised papers and 23 poster papers presented together with 3 invited talks and 2 tutorial papers were carefully reviewed and selected from 67 submissions. The papers are organized in topical sections on Euler and Venn diagrams, formal aspects of diagrams, reasoning with diagrams, interacting with diagrams, constructing diagrams, and understanding diagrams and text.