This handbook is a selection of foundational aspects, security analysis, platforms, and applications of blockchains that consists of four parts. The first part introduces the basic building blocks such as distributed computing and cryptography. Consensus algorithms that form the basic backbone of blockchain protocols are presented. Various cryptographic tools like hash functions, digital signatures and commitment schemes are also introduced. Advanced cryptographic techniques such as zero knowledge protocols, secret sharing, verifiable random functions that are used for privacy-preserving and secure design are discussed. The second part of this handbook consists of popular blockchain designs and platforms. Architecture of Bitcoin, Ethereum, Monero, Tendermint and Algorand have been presented. Various important issues like scalability and security are discussed in the third part. Security design challenges, security vulnerabilities and their analysis are discussed. The final part of this handbook discusses various applications of blockchains. These include supply-chain, identity and credential management, Internet of Things (IoT), data-sharing, e-voting, e-governance, e-health, smart cities, and Industry 4.0. Research challenges and directions of future work are included in this handbook. This comprehensive reference targets students and researchers, who are starting to explore blockchain. Professionals working in blockchain security and applications will find this handbook to be a valuable reference.