Bluesky is a decentralized social network built on the AT Protocol, a decentralized foundation for public social media. Launched in private beta in February 2023, it has grown to over 3 million registered users in 2024. The AT Protocol is designed to allow multiple interoperable providers for every part of the system, making it easy for users to switch providers. Bluesky aims to provide a user experience of the same or better quality than centralized services while being open and decentralized. The system's openness allows anyone to contribute to content moderation and community management. Bluesky allows users to choose which moderation services they want to subscribe to, and moderation services are decoupled from hosting providers, making it easy for users to switch moderation services. The AT Protocol is designed to support multiple social modes, not just Bluesky. It can be used to implement Reddit-style forums, long-form blogs, or domain-specific social applications. The AT Protocol provides cryptographically authenticated data, but our implementation pairs it with custodial key management to provide a familiar user experience. The system is open to third-party clients and alternative PDS hosts, and anyone can index the network using real-time streams for low-latency updates. This reduces the dependency on any one provider, since every part of the system can be run by multiple competing providers, and users can switch providers with minimal friction. The AT Protocol is also extensible to other social modes besides microblogging.Bluesky is a decentralized social network built on the AT Protocol, a decentralized foundation for public social media. Launched in private beta in February 2023, it has grown to over 3 million registered users in 2024. The AT Protocol is designed to allow multiple interoperable providers for every part of the system, making it easy for users to switch providers. Bluesky aims to provide a user experience of the same or better quality than centralized services while being open and decentralized. The system's openness allows anyone to contribute to content moderation and community management. Bluesky allows users to choose which moderation services they want to subscribe to, and moderation services are decoupled from hosting providers, making it easy for users to switch moderation services. The AT Protocol is designed to support multiple social modes, not just Bluesky. It can be used to implement Reddit-style forums, long-form blogs, or domain-specific social applications. The AT Protocol provides cryptographically authenticated data, but our implementation pairs it with custodial key management to provide a familiar user experience. The system is open to third-party clients and alternative PDS hosts, and anyone can index the network using real-time streams for low-latency updates. This reduces the dependency on any one provider, since every part of the system can be run by multiple competing providers, and users can switch providers with minimal friction. The AT Protocol is also extensible to other social modes besides microblogging.