Security and privacy in blockchain technology is a critical research area that focuses on protecting data integrity, confidentiality, and user anonymity in decentralized networks. While blockchains provide inherent transparency and immutability, they are susceptible to attacks such as double spending, 51% attacks, Sybil attacks, smart contract vulnerabilities, and privacy leaks. Early research explored cryptographic foundations, including hash functions, digital signatures, and public-private key infrastructures, while recent studies investigate advanced techniques such as zero-knowledge proofs, homomorphic encryption, ring signatures, and secure multi-party computation to enhance privacy and confidentiality. Research also addresses secure consensus mechanisms, intrusion detection, access control, and blockchain-based identity management systems. Applications span cryptocurrencies, decentralized finance (DeFi), healthcare, IoT networks, supply chain management, and secure data sharing. Current studies emphasize balancing transparency, decentralization, and privacy, as well as improving resilience against emerging attacks, regulatory compliance, and scalability, establishing security and privacy as essential pillars for trustworthy and robust blockchain systems.