This book provides a snapshot of the current state of the art related to the development of tools, frameworks and techniques for designing and implementing multi-agent systems. The single contributions are diverse in nature, with some providing overviews of toolkits, while others focus on specific features or application domains. To reflect this diversity, the book has been organized into two parts. The first part covers Languages, Frameworks and Platforms and contains six chapters that present overviews of some of the new toolkits that have emerged, and reflections on the evolution of some of the existing toolkits. The second part contains four chapters and covers Features, Mechanisms and Applications that have been developed for and with various agent toolkits. These chapters provide a more focused description of a specific mechanism or feature of a toolkit, or they describe toolkits that have been developed for a specific application domain. All the chapters include the rationale for the need of the toolkit or mechanism described, how it is positioned in the scientific panorama particularly in relation to the most recent advancements in artificial intelligence, a technical description, an analysis of limitations, and ethical and technical challenges. Representing the results of the most recent research on engineering agents and multi-agent systems, the book mainly targets researchers. In addition, it will also be useful for advanced professionals in industry, as the book may also serve as a roadmap to select the best tool and approach for a given problem.