This book constitutes revised selected papers from the 9th International Workshop on Engineering Multi-Agent Systems, EMAS 2021, which was held during May 3-4, 2021. The conference was initially planned to take place in London, UK, but changed to an online event due to the COVID-19 pandemic.

PanSim + Sim-2APL: A Framework for Large-Scale Distributed Simulation with Complex Agents.- Implementing Ethical Governors in BDI.- A Unifying Framework for Agency in Hypermedia Environments.- Multiagent Foundations for Distributed Systems: A Vision.- An Epistemic Logic for Modular Development of Multi-Agent Systems.- Attention Guidance Agents with Eye-tracking: A Use-case Based on the MATBII Cockpit Task.- StreamB: A Declarative Language for Automatically Processing Data Streams in Abstract Environments for Agent Platforms.- BDI for Autonomous Mobile Robot Navigation.- An Appraisal Transition System for Event-driven Emotions in Agent-based Player Experience Testing.- Developer Operations and Engineering Multi-Agent Systems.- Smart Cyber-physical System-of-Systems using Intelligent Agents and MAS.- Formal Verification of a Map Merging Protocol in the Multi-Agent Programming Contest.- Analysis of the Execution Time of the Jason BDI Reasoning Cycle.- Autonomous Economic Agent Framework.-Seamless Integration and Testing for MAS Engineering.- Engineering Explainable Agents: An Argumentation-Based Approach.- TPO: A Type System for the Architecture of Agent Societies.- A Practical Framework for General Dialogue-based Bilateral Interactions.- Implementing Durative Actions with Failure Detection in Gwendolen.- Concept Description and Definition Extraction for the ANEMONE System.- GenGPT: a Systematic Way to Generate Synthetic Goal-Plan Trees.