In this episode we will review our research on 51% attacks on Proof-of-Work (PoW) blockchains, where an attacker gains control of over half the network's computing power to manipulate transactions. It explains that while large cryptocurrencies like Bitcoin are largely secure due to immense hash rates, smaller altcoins remain highly vulnerable, often due to the availability of hash rate rental services that lower attack costs. The sources detail various attack motivations, from financial gain (double-spending) to non-financial objectives like network sabotage. Finally, the text explores defense mechanisms, including strengthening decentralization, protocol enhancements like checkpointing, and the potential transition to Proof-of-Stake (PoS) to enhance security.