Blockchains all have at their core certain key-characteristics, such as the fact that the system is decentralized and distributed, immutable, and transparent. The consequences of such features are twofold with regard to data protection and privacy. On the one hand, some of these features are at odds with the way we understand privacy in Europe and the fundamental legal principles governing data protection law in this legal order. Indeed, the key features of blockchain systems may be in quite a stark contrast with some fundamental privacy rights under the GDPR, such as for instance the rights of access, rectification or erasure. On the other hand, blockchains may also constitute an opportunity for the protection of personal data, in limiting or even vanishing ‘single point of failure’ issues, alteration of data or data thefts, and increasing security and transparency. In this chapter, the authors analyze the intersection between blockchains and data protection regulations (in particular the GDPR), the good and the bad that emanate from this necessary relationship, and they explore possible solutions which can be implemented.