This chapter addresses collusive agreements on blockchain, meaning collusion that is either enabled by blockchain ecosystems. The challenges that authorities face in this area are daunting. Collusive behavior harms consumer welfare by artificially concentrating the market in the hands of a few. My ambition is to put forward a framework that would help decision-makers understand and address that idiosyncratic competitive risks to which blockchain collusion may give rise (Section 1). I first analyze whether the creation of a blockchain could, in and of itself, constitute an infringement of antitrust law (Section 2) and whether existing blockchains might give rise to distinct forms of collusion (Section 3).