Sybil attacks, named after the 1973 book "Sybil," exploit peer-to-peer networks, particularly decentralized blockchain systems, by using multiple fake identities to manipulate or control a significant portion of network nodes. First defined in the early 2000s by Microsoft researchers, these attacks can lead to severe consequences such as 51% attacks on blockchains or misinformation spread on social media. Two primary types of Sybil attacks are direct, where malicious nodes interact with honest ones to influence decisions, and indirect, where reputation is manipulated without direct interaction. Blockchain networks are particularly vulnerable due to their pseudoanonymous nature, but they also incorporate resilience mechanisms such as cryptoeconomic security and reputation systems to mitigate these threats. While identity verification is a robust defense, it is often impractical for permissionless networks. As blockchain technology advances, new strategies and innovations continue to evolve to enhance resilience against Sybil attacks, ensuring the open and permissionless nature of these systems is maintained.