This course introduces blockchain security, including a description of how the blockchain works at each level of the blockchain ecosystem. The instructor begins with the building blocks that create the structure of blockchain, the cryptography that it uses for security, and the role of hash functions in the blockchain and how they can be attacked. In the next module, the instructor describes what blockchain consensus is, why it’s needed, its underlying theory (Byzantine Fault Tolerance and Security via Scarcity), some of the common consensus algorithms, and the security issues inherent to each variant. The course progresses with a module that describes how blocks are created, the nodes and network that make up the blockchain ecosystem, and examples of various attacks that can be made on their security. Next the focus is on smart contracts and how their security can be compromised with vulnerabilities created by common programming errors during contract development, including in Ethereum, the most commonly used smart contract platform. Finally, the course wraps up with discussions of distributed ledger architectures that are alternatives to blockchain, second-level blockchain protocols, and advanced cryptography in blockchain.