This thesis examines the security, performance, and privacy of Proof of Work (PoW)-based blockchains and digital currencies like Bitcoin. While the decentralized nature of blockchains removes the need for trusted third parties, it introduces new challenges in security, performance, scalability, and privacy. The security of a blockchain directly affects participants' ability to exchange value or engage in network communication and consensus. In the first contribution, we observe the decentralized nature of Bitcoin and show that few individuals typically control vital operations in the Bitcoin ecosystem. Moreover, we demonstrate that a third party can unilaterally affect the fungibility of individual Bitcoins. Our second contribution provides a quantitative framework to objectively compare the security and performance characteristics of PoW-based blockchains under adversaries with optimal strategies. Our work allows us to increase Bitcoin's transaction throughput by a factor of ten with only one parameter change, without compromising the security of the underlying blockchain. In the third contribution, we highlight previously unconsidered impacts of PoW blockchain scalability on its security and propose design modifications now implemented in the primary Bitcoin client. In the fourth contribution, we investigate the privacy of lightweight Bitcoin clients, which are critical for mainstream adoption. We also propose appropriate design modifications to protect user privacy. Orthogonally, in the fifth contribution, we analyze the location privacy implications of public transaction prices. Surprisingly, we show that with only a few prices from a consumer, we can accurately determine the purchase location.This thesis examines the security, performance, and privacy of Proof of Work (PoW)-based blockchains and digital currencies like Bitcoin. While the decentralized nature of blockchains removes the need for trusted third parties, it introduces new challenges in security, performance, scalability, and privacy. The security of a blockchain directly affects participants' ability to exchange value or engage in network communication and consensus. In the first contribution, we observe the decentralized nature of Bitcoin and show that few individuals typically control vital operations in the Bitcoin ecosystem. Moreover, we demonstrate that a third party can unilaterally affect the fungibility of individual Bitcoins. Our second contribution provides a quantitative framework to objectively compare the security and performance characteristics of PoW-based blockchains under adversaries with optimal strategies. Our work allows us to increase Bitcoin's transaction throughput by a factor of ten with only one parameter change, without compromising the security of the underlying blockchain. In the third contribution, we highlight previously unconsidered impacts of PoW blockchain scalability on its security and propose design modifications now implemented in the primary Bitcoin client. In the fourth contribution, we investigate the privacy of lightweight Bitcoin clients, which are critical for mainstream adoption. We also propose appropriate design modifications to protect user privacy. Orthogonally, in the fifth contribution, we analyze the location privacy implications of public transaction prices. Surprisingly, we show that with only a few prices from a consumer, we can accurately determine the purchase location.