This book constitutes the thoroughly refereed post-proceedings of the 12th International Conference on Computer Aided Systems Theory, EUROCAST 2009, held in Las Palmas de Gran Canaria, Spain in February 2009. The 120 revised full papers presented were carefully reviewed and selected for inclusion in the book. The papers are organized in topical sections on systems theory and simulation: formal approaches, computation and simulation in modeling biological Systems, intelligent information processing, applied formal verification, computer vision and image processing, mobile and autonomous systems: robots and cars, simulation based system optimization, signal processing methods in systems design and cybernetics, polynomial models in control system design, heurist problem solving, simulation and formal methods in systems design and engineering, models of co-operative engineering systems.