Block stuffing is a form of denial-of-service attack where malicious actors flood blockchains with low-value transactions to consume blockspace. This causes delays, raises fees, and can disrupt user activity, MEV strategies, or on-chain governance. Common on Ethereum, Cosmos, and Solana, it exploits the economic limits of block capacity. While defenses exist—like EIP-1559, spam filtering, and rate limits—block stuffing remains a persistent challenge. Understanding its mechanics helps developers, validators, and users build and support more resilient blockchain ecosystems.