Bluesky is a decentralized social network built on the AT Protocol, launched in private beta in February 2023 and growing to over 3 million registered users by January 2024. The paper introduces the architecture of Bluesky and the AT Protocol, which aims to provide a user experience similar to centralized services while being decentralized. Key features include multiple interoperable providers, ease of switching providers, user control over content, and a simple user interface. The AT Protocol supports various social modes, with user data stored in repositories that are cryptographically authenticated and accessible via Personal Data Servers (PDSes). The system includes indexing infrastructure for real-time updates and content discovery, and allows for third-party moderation services and feed generators. Bluesky emphasizes user agency and a pluralistic ecosystem, where users can choose their moderation preferences and content filtering. The paper also discusses related work and concludes by highlighting Bluesky's approach to addressing problematic content and its open architecture.Bluesky is a decentralized social network built on the AT Protocol, launched in private beta in February 2023 and growing to over 3 million registered users by January 2024. The paper introduces the architecture of Bluesky and the AT Protocol, which aims to provide a user experience similar to centralized services while being decentralized. Key features include multiple interoperable providers, ease of switching providers, user control over content, and a simple user interface. The AT Protocol supports various social modes, with user data stored in repositories that are cryptographically authenticated and accessible via Personal Data Servers (PDSes). The system includes indexing infrastructure for real-time updates and content discovery, and allows for third-party moderation services and feed generators. Bluesky emphasizes user agency and a pluralistic ecosystem, where users can choose their moderation preferences and content filtering. The paper also discusses related work and concludes by highlighting Bluesky's approach to addressing problematic content and its open architecture.