This book constitutes the thoroughly refereed postproceedings of the 9th International Workshop on Multi-Agent-based Simulation, MABS 2008, held in Estoril, Portugal, in May 2008. The 16 revised full papers presented have gone through two rounds of reviewing, selection, and improvement and were selected from 44 submissions; they present state-of-the-art research results in agent-based simulation and modeling. The papers are organized in topical sections on simulation of economic behaviour; modelling and simulation of social behaviou; applications; techniques, infrastructure and technologies as well as methods and methodologies.