Facebook's web site and mobile apps are very dynamic. Every day, they undergo thousands of online configuration changes, and execute trillions of configuration checks to personalize the product features experienced by hundreds of million of daily active users. For example, configuration changes help manage the rollouts of new product features, perform A/B testing experiments on mobile devices to identify the best echo-canceling parameters for VoIP, rebalance the load across global regions, and deploy the latest machine learning models to improve News Feed ranking. This paper gives a comprehensive description of the use cases, design, implementation, and usage statistics of a suite of tools that manage Facebook's configuration end-to-end, including the frontend products, backend systems, and mobile apps.