Modular blockchains are a new paradigm in blockchain design. Instead of one blockchain doing everything, modular blockchains specialize and optimize to perform a given function. This specialization provides breakthroughs in scalability, flexibility, and interoperability, enabling developers to build blockchain applications for mass adoption. Celestia is the first modular blockchain network. By decoupling consensus from execution, Celestia enables anyone to easily deploy their own blockchain, without the overhead of bootstrapping a new consensus network. Blockchains on Celestia are simultaneously scalable, sovereign and secure.