This thesis, submitted by Arthur Gerais for the degree of Doctor of Sciences at ETH Zurich, examines the security, performance, and privacy of Proof of Work (PoW) blockchains and digital currencies like Bitcoin. The decentralized nature of blockchains removes trusted third parties but introduces new challenges in security, performance, scalability, and privacy. The thesis makes several key contributions: 1. **Decentralization and Control**: It observes that a few individuals control vital operations in the Bitcoin ecosystem and that a third party can unilaterally affect the fungibility of individual Bitcoins. 2. **Quantitative Framework**: It provides a quantitative method to compare the security and performance of PoW blockchains under optimal adversary strategies, enabling a tenfold increase in Bitcoin's transaction throughput without compromising security. 3. **Scalability and Security**: It highlights the previously overlooked impacts of PoW blockchain scalability on security and proposes design modifications implemented in the primary Bitcoin client. 4. **Privacy of Lightweight Clients**: It investigates the privacy of lightweight Bitcoin clients, which are crucial for Bitcoin's mainstream adoption, and suggests design changes to protect user privacy. 5. **Location Privacy**: It analyzes the implications of public transaction prices on location privacy, showing that with a few consumer purchase prices, the exact location can be accurately determined. These contributions aim to enhance the security, performance, and privacy of PoW blockchains and Bitcoin.This thesis, submitted by Arthur Gerais for the degree of Doctor of Sciences at ETH Zurich, examines the security, performance, and privacy of Proof of Work (PoW) blockchains and digital currencies like Bitcoin. The decentralized nature of blockchains removes trusted third parties but introduces new challenges in security, performance, scalability, and privacy. The thesis makes several key contributions: 1. **Decentralization and Control**: It observes that a few individuals control vital operations in the Bitcoin ecosystem and that a third party can unilaterally affect the fungibility of individual Bitcoins. 2. **Quantitative Framework**: It provides a quantitative method to compare the security and performance of PoW blockchains under optimal adversary strategies, enabling a tenfold increase in Bitcoin's transaction throughput without compromising security. 3. **Scalability and Security**: It highlights the previously overlooked impacts of PoW blockchain scalability on security and proposes design modifications implemented in the primary Bitcoin client. 4. **Privacy of Lightweight Clients**: It investigates the privacy of lightweight Bitcoin clients, which are crucial for Bitcoin's mainstream adoption, and suggests design changes to protect user privacy. 5. **Location Privacy**: It analyzes the implications of public transaction prices on location privacy, showing that with a few consumer purchase prices, the exact location can be accurately determined. These contributions aim to enhance the security, performance, and privacy of PoW blockchains and Bitcoin.