This volume puts together most of the papers given at the interdisciplinary workshop on DIAGRAMMATOLOGY AND DIAGRAM PRAXIS held at the University of Lisbon, 23-24th March 2009. The workshop was organized by the Research Project "Image in Science and Art" of the Center for Philosophy of Science of the University of Lisbon (CFCUL), aiming to analyze the place of image and diagrammatic thinking in different epistemological and semiotic programs. The articles are organized in three main sections. The first, Fundamental Issues of Diagrammatology, is more historically oriented including papers from Olga Pombo, Sybille Krämer, Jan Wöpking, Frederik Stjernfelt and Ahti-Veikko Pietarinen. The second part, Visual Diagram Praxis, comprises four case studies on diagram practices from Augusto J. Franco Oliveira, Ángel Nepomuceno-Fernández, J.R. Croca and Matthias Bauer. Finally, the third part, On Maps, consists of studies by Valeria Giardino, Alexander Gerner, Christoph Ernst and Catarina Pombo Nabels.