Layer 2 (L2) solutions are off-chain scaling frameworks built on top of Layer 1 blockchains like Ethereum and Bitcoin to solve the blockchain trilemma of scalability, security, and decentralization. By moving transaction execution off the congested base layer, L2s enable high throughput, lower fees, and faster confirmation times while still anchoring security and finality back to the underlying L1. The main L2 architectures include Rollups (Optimistic & ZK-Rollups), which bundle thousands of transactions and post compressed proofs on-chain, State Channels for instant peer-to-peer updates, and Sidechains, which run parallel to L1 with independent validators. Optimistic Rollups rely on fraud proofs and challenge periods, while ZK-Rollups use zero-knowledge validity proofs for faster withdrawals and better privacy. This modular approach allows DeFi, NFT marketplaces, gaming, and high-frequency trading dApps to scale without sacrificing trustlessness. Popular ecosystems like Arbitrum, Optimism, Polygon zkEVM, and zkSync demonstrate how L2s unlock mass adoption by making decentralized applications as seamless and cost-efficient as Web2 apps.