DARPA is accepting proposals using artificial intelligence (AI) to create realistic environments and train cyber agents to counter advanced persistent cyber threats. The Cyber Agents for Security Testing and Learning Environments (CASTLE) program seeks to improve cyber testing and evaluation by developing a toolkit that instantiates realistic network environments and trains AI agents to defend against advanced persistent cyber threats (APTs). Teams will use a class of machine learning known as reinforcement learning to automate the process of reducing vulnerabilities within a network.