This technical report, authored by a large group of researchers from various institutions and edited by Lewis Hammond from the Cooperative AI Foundation, examines the risks inherent in multi-agent AI systems. It provides a structured taxonomy by identifying three primary failure modes: miscoordination, conflict, and collusion, which arise from agent incentives. Seven key risk factors are also discussed, including information asymmetries, network effects, and emergent agency, which can underpin these failures. The report grounds its analysis in real-world examples and experimental evidence, highlighting distinct challenges for AI safety, governance, and ethics posed by these complex systems. Finally, the authors offer potential mitigation strategies and directions for future research.