Lending Markets from First Principles

Most people meet DeFi lending through a brand: "I deposit on Aave," "I borrow on Compound." The brands obscure a simple structure underneath. An on-chain money market is a smart contract that holds a pool of one asset (say USDC), lets one set of users supply that asset to the pool, and lets a separate set of users post unrelated collateral (say ETH) to borrow from the pool. The contract continuously computes an interest rate based on how much of the pool is currently borrowed, charges that rate to borrowers, and credits a (lower) share of it to suppliers. There is no loan officer, no credit check, no maturity date, and no counterparty matching. The pool itself is the counterparty. This pooled-liquidity design is the central innovation. Traditional peer-to-peer lending requires someone to want to lend exactly what someone else wants to borrow, for exactly the duration they need it. That coincidence-of-wants problem killed early P2P crypto lending products. Pool-based money markets eliminated it by making every supplier interchangeable: you do not lend to a specific borrower, you lend to the pool, and the pool lends to whoever shows up with collateral. Your funds are immediately fungible with every other supplier's funds, and you can withdraw at any moment (subject to a utilization constraint we will get to). Because there is no credit check, every loan has to be **overcollateralized**. If you want to borrow $1,000 of USDC, you have to post substantially more than $1,000 of some other asset as collateral. The protocol does not care who you are; it only cares that if your collateral value falls, it can liquidate it for enough to repay the loan plus a buffer. This is the entire trust model: not "we trust the borrower" but "we trust the math on the collateral." The rest of this lesson is about how the math works — interest rates, collateral factors, liquidation engines — and how the two largest implementations (Aave and Compound) actually deploy that math in production.