This book constitutes the refereed proceedings of the 12th International Workshop on Multi-Agent-Based Simulation, MABS 2011, held in Taipei, Taiwan, in May 2011. The 10 revised selected and extended papers presented were carefully reviewed and selected from 21 submissions. The topics covered are MABS in scientometrics; MABS in politics, transit and policy; pedestrian, crowds and large scale MABS; and MABS modeling.

Based on two international workshops on trust in agent societies, held at AAMAS 2003 and AAMAS 2004, this book draws together carefully revised papers on trust, reputation, and security in agent society. Besides workshop papers, several contributions from leading researchers in this interdisciplinary field were solicited to complete coverage of all relevant topics.

The 13 papers presented take into account issues from multiagent systems, artificial intelligence, cognitive science, game theory, and social and organizational science. Theoretical topics are addressed as well as applications in human-computer interaction and e-commerce.

This special issue is the result of the selection and re-submission of advanced and revised versions of papers from the workshop on "Trust in Agent Societies" (11th edition), held in Estoril (Portugal) on May 10, 2008 as part of the Autonomous Agents and Multi-Agent Systems 2008 Conference (AAMAS 2008), and organized by Rino Falcone, Suzanne Barber, Jordi Sabater-Mir, and Munindar Singh. The aim of the workshop was to bring together researchers from different fields (artificial intelligence, multi-agent systems, cognitive science, game theory, and social and organizational sciences) that could contribute to a better understanding of trust and reputation in agent societies. The workshop scope included theoretical results as well their applications in human-computer interaction and electronic commerce. It was constituted by a main session integrated with two others: the first on the formal models of trust, and the second on reputation models. In this volume we present papers from the three workshop sessions: the main s- sion with papers on theoretical and applicative aspects of trust (from a engineering, cognitive, computational, sociological point of view); the formal model session with works in the field of applied logic and applied mathematics; the reputation models session with papers that specifically address models of reputation systems, theo- driven and empirically backed-up guidelines for designing reputation technologies, and analysis and discussion of existing reputation systems.