The Genesis Block is the inaugural block in any blockchain network, forming the bedrock upon which all subsequent blocks are built. It establishes the network's initial rules and data, setting the stage for the blockchain's operation. In Bitcoin, the Genesis Block was mined on January 3, 2009, famously containing a message about the 2008 financial crisis. Ethereum's Genesis Block, launched in 2015, was pivotal in introducing smart contracts, facilitating the creation of decentralized applications.