Concordia is a library developed by Google DeepMind designed for generative social simulation, enabling the construction and use of agent-based models that simulate interactions among agents in various environments, whether physical, social, or digital. The library employs a unique interaction pattern reminiscent of tabletop role-playing games, where a special agent known as the Game Master (GM) orchestrates the simulation. The GM acts as a narrator, interpreting the actions of player agents, which are expressed in natural language, and translating these actions into practical implementations within the simulated environment. In a physical simulation, the GM assesses the plausibility of agent actions and describes their consequences. In digital contexts, the GM may facilitate necessary API calls to integrate with external tools based on agent inputs. This flexibility allows Concordia to be applied across diverse fields, including social science research, AI ethics, cognitive neuroscience, and economics. It can also generate data for personalization applications and evaluate the performance of real services through simulated usage. To utilize Concordia, users need access to a standard Large Language Model (LLM) API, which is essential for generating responses and actions. The library can be installed via the Python Package Index (PyPI) using a simple pip command, or users can opt for a manual installation if they wish to work directly with the source code. The installation process includes cloning the repository and setting it up in an editable mode for development purposes. An illustrative example of Concordia's capabilities involves a social simulation scenario where four friends are trapped in a snowed-in pub, with two of them embroiled in a dispute over a crashed car. The agents in this simulation are designed based on a reasoning framework that prompts them to consider their situation, their identity, and appropriate actions in response to their circumstances. For those who wish to cite Concordia in their work, a specific article detailing its methodology and applications is provided for reference. It is important to note that Concordia is not an officially supported Google product, but it offers a robust framework for researchers and developers interested in exploring generative agent-based modeling.