This Chapter considers the legal framework of blockchain “communities”. This term refers collectively to the various groups of participants to blockchain projects, spanning from coders to miners, from users to investors, from developers to stakeholders. Such a diverse series of interest-bearers needs to have a functioning legal framework in order to accommodate the different needs, balance or choose among them, and solve potential disputes. The Chapter illustrates such need for rules, and first reflects on which would be the most preferable source for them, be it a purely contractual one, resulting in a fully decentralised model, or instead a hard law approach, reducing the number of available options for operators in order to pursue broader goals, or some middle way between these two opposites. It is argued that the former approach is the preferable one. The ideal self-regulatory approach identified is then contrasted with the reality of how decisions are normally taken in the day-to-day activities of blockchain-related communities. After that, within the broader picture of the governance of blockchain communities, the Chapter investigates the specific problem of the legal nature of 51% attacks. One could wonder if they are even an illegal act: they happen within a community that accepts certain rules of the game, and among those rules there is the one according to which decisions are made with an agreement of 51% of the nodes. Is acquiring such 51% illegal? If so, under what rules? Is it competition law? Is it other rules? The conclusion summarises the analysis and offers some perspective (and especially some caveats) for regulators worldwide, in an attempt to prevent an over-regulation of blockchain communities that would risk slowing down and discouraging innovation.