This article delves into the privacy protection issues in blockchain technology, emphasizing the importance of privacy as a fundamental human right for freedom and democracy. It provides a detailed introduction to Privacy-Enhancing Technologies (PETs), including zero-knowledge proofs, multi-party computation, fully homomorphic encryption, and trusted execution environments. The article analyzes these technologies in terms of their generality, composability, computational efficiency, network efficiency, degree of decentralization, and cost. It also discusses practical application cases for these technologies and explores how hybrid approaches can leverage the complementary strengths of different techniques.