Smart contracts are programs that execute on blockchains. Their strong security properties (e.g., transparency, tamper-resistance, and censorship-resistance) have attracted significant attention and investment ($31B via ICOs as of 2019), but most of their real-world uses, such as tokens, exercise little of smart contracts' potential power. A key reason is a disconnection from the real world: There is currently no secure, decentralized way to faithfully convey real-world states to blockchains. Worse yet, smart contracts inherit blockchains' lack of confidentiality and poor efficiency. This thesis introduces solutions to these problems by connecting blockchains with off-chain systems. The systems presented in this thesis advance the state of the art of smart contract capabilities. Specifically, this thesis explores three research directions: (1) authenticated data oracles that enable faithful representation of real-world states on blockchains. (2) decentralized secret storage that endows smart contracts with privacy by storing secrets and performing computation in off-chain committees. (3) resource-efficient consensus that achieves Proof of Work style consensus while avoiding wasteful computation. Beyond the scholarly contributions, several works in this thesis have seen industry adoption.