The connection between Byzantine fault tolerance and cryptocurrencies, such as Bitcoin, may not be apparent immediately. Byzantine fault tolerance is intimately linked to engineering and design challenges of developing long-running and safety-critical technical systems. Its origins can be traced back to the question of how to deal with faulty sensors in distributed systems and the fundamental insight that majority voting schemes may be insufficient to guarantee correctness if arbitrary, or so-called Byzantine failures, can occur. However, achieving resilience against Byzantine failures has its price, both in terms of the redundancy required within a system and the incurred communication overhead. Together with the complexity of correctly implementing Byzantine fault-tolerant (BFT) protocols, it may help to explain why BFT systems have not yet been widely deployed in practice, even though practical designs exist for almost 20 years. On the other hand, asking anyone about Bitcoin or blockchain 10 years ago would have only raised quizzical looks. Since then, the ecosphere surrounding blockchain technologies has grown from the pseudonymously published proposal for a peer-to-peer electronic cash system into a multi-billion-dollar industry. At the heart of this success story lies not only the technical innovations presented by Bitcoin but a colorful and diverse community that has succeeded in bridging gaps and bringing together various disciplines from academia and industry alike. Bitcoin reinvigorated interest in the topic of BFT as it was arguably the first system that achieved a practical form of Byzantine fault tolerance with a large and changing number of participants. Research into the fundamental principles and mechanisms behind the underlying blockchain technology of Bitcoin has since helped advance the field and state of the art regarding BFT protocols. This chapter will outline how these modern blockchain technologies relate to the field of Byzantine fault tolerance and outline advantages and disadvantages in their design decisions and fundamental assumptions. Thereby, we highlight that Byzantine fault tolerance should be considered a practical and fundamental building block for modern long-running and safety critical systems and that the principles, mechanisms, and blockchain technologies themselves could help improve the security and quality of such systems.

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