This chapter provides a comprehensive exploration of the blockchain security landscape by dividing it into three critical aspects: process security, data security, and infrastructure security. It examines the inherent security features of blockchain, such as decentralization, cryptographic hashing, and consensus mechanisms, while also discussing its vulnerabilities to various threats like 51% attacks, Sybil attacks, and smart contract exploits. The chapter delves into modern cybersecurity techniques, including AI-driven threat detection, Zero Trust models, and quantum-resistant cryptography, highlighting the increasing importance of these solutions in securing blockchain systems, particularly in healthcare. Additionally, the chapter explores blockchain classification based on architecture (public, private, consortium), consensus mechanisms (PoW, PoS, DPoS), and applications in industries such as finance, healthcare, and supply chain management.