How Lending and Borrowing Protocols Work in DeFi

DeFi promises to replace banks with code. What does that actually look like in practice? You get lending protocols that operate 24/7, ask for no identity verification, and let anyone with a cryptocurrency wallet become both lender and borrower. The mechanics are more elegant than the jargon suggests—and understanding them is essential before you stake your first dollar equivalent in any DeFi market.

This guide covers how lending and borrowing protocols actually work. You’ll learn why your deposited crypto earns interest, how borrowers maintain their collateral, what triggers liquidations, and which protocols dominate the space in early 2025.

The Supply Side: How Lending Actually Works

When you deposit cryptocurrency into a DeFi lending protocol like Aave or Compound, you’re not simply storing assets. You’re supplying them to a pooled liquidity market that borrowers draw from. Your deposit becomes part of a smart contract—a piece of code running on the blockchain that automates every aspect of the transaction.

Here’s the concrete flow: you approve a token transfer, your assets move into the protocol’s contract, and the protocol mints you equivalent tokens called aTokens (on Aave) or cTokens (on Compound). These represent your share of the lending pool. The tokens accumulate value over time as interest accrues, which is why your balance increases automatically. There’s no bank statement, no monthly payout—everything happens mathematically within the contract.

The interest you earn comes from borrowers paying interest on their loans, which gets distributed proportionally to all suppliers. This is the fundamental engine: borrowers pay, lenders earn. The rate fluctuates based on utilization—the percentage of the pool that is currently borrowed. When demand is high, rates climb. When the pool sits idle, they fall. Aave and Compound both use algorithmic interest rate models that adjust dynamically, though they differ in their specific formulas.

A practical example: in late 2024, USDC supply rates on Aave hovered around 3-4% annually during periods of normal utilization. That rate wasn’t set by any committee. It emerged from the protocol’s algorithm responding to how much capital was being borrowed versus supplied.

The Borrow Side: Getting Crypto Against Your Crypto

Borrowing in DeFi works differently than traditional finance. You don’t borrow fiat currency against your paycheck or credit history. Instead, you deposit cryptocurrency as collateral and borrow a different cryptocurrency against that collateral. The system is overcollateralized by design—borrowers must put up more value than they take out.

The key metric here is the collateral factor, sometimes called the loan-to-value ratio. Each asset has its own collateral factor determined by the protocol’s risk assessment. On Aave, ETH typically carries an 80-85% collateral factor, meaning you can borrow up to 80-85% of your ETH deposit’s value in another asset. More volatile or less liquid assets have lower factors—some tokens might only allow 40% or lower.

But collateral alone isn’t enough. Protocols calculate a health factor for each borrower’s position. This number represents the health of your loan: above 1 means you’re solvent; below 1 triggers liquidation risk. The health factor considers your total borrowed value, your collateral value, and the collateral factor of each asset. When you borrow against multiple assets or multiple positions, the calculation becomes a weighted average across all your deposits.

Consider this scenario: you deposit 10 ETH worth $30,000 (at $3,000 per ETH) and borrow 15,000 USDC. With an 80% collateral factor, your maximum borrow would be $24,000. Your health factor starts comfortable. But if ETH drops to $2,400, your collateral is now worth $24,000, your borrowed amount stays at $15,000, and your health factor drops precipitously. This is where liquidation becomes imminent.

Interest Rates: The Market Mechanism

Both supply and borrow rates exist in DeFi lending, and they’re mathematically linked. The borrow rate is always higher than the supply rate—the difference is the protocol’s revenue, sometimes called the spread or reserve factor.

The interest rate model typically uses a sigmoid or piecewise linear function. When utilization is low, rates rise slowly to encourage borrowing. When utilization climbs toward critical levels (often 80-90%), rates spike aggressively to deter further borrowing and incentivize repayment. This is the market mechanism attempting to maintain liquidity reserves.

On Compound, the utilization rate directly determines the borrow rate using a formula that combines a base rate, a slope, and the utilization multiplier. On Aave, the model is more complex, with different parameters for different asset tiers and a variable rate that shifts based on pool utilization.

The practical implication: as a supplier, you want high utilization (more demand for loans means higher rates for you). As a borrower, you want low utilization (cheaper rates). These competing interests create dynamic markets. In practice, supply rates on major assets like USDC and ETH tend to track between 2-8% annually, while borrow rates run 3-12%, with the spread varying by asset risk and market conditions.

Liquidation: The Safety Valve

Liquidations are what happen when the system needs to recover its money. They’re not a penalty—they’re a mechanism that keeps the protocol solvent by ensuring every loan is backed by sufficient collateral at all times.

When a borrower’s health factor drops below 1, anyone can trigger liquidation. Liquidators repay part or all of the outstanding loan and receive the borrower’s collateral at a discount—typically 5-10% below market price. This discount is the incentive for liquidators to participate. They profit by buying collateral cheaply and immediately selling it (or holding it) at market rates.

The liquidation threshold is slightly different from the collateral factor. On Aave, there’s a liquidation threshold (often 5% higher than the collateral factor) that determines when liquidation becomes possible. This buffer exists to give borrowers warning and time to add collateral or repay debt before full liquidation occurs.

In practice, liquidations happen frequently during volatile market periods. During the crypto market turbulence of early 2024, Aave processed millions in liquidated positions across multiple asset classes. These events are visible on-chain—the transactions are public, and analytics platforms like DeFiLlama and Zapper track them in real-time. Understanding that liquidations are normal, expected, and mechanized helps demystify them.

Major Protocols: Aave and Compound

The DeFi lending landscape centers on two protocols that pioneered the model: Aave and Compound. Both have billions in total value locked, but they differ in important ways.

Compound, founded by Robert Leshner and Geoffrey Hayes, launched in 2018 and pioneered the algorithmic interest rate model. Its governance is relatively conservative, and the protocol focuses on stability and simplicity. Compound V3, deployed in 2023, introduced isolated markets that allow new assets to be added with custom risk parameters without affecting the entire system.

Aave, originally called ETHLend, rebranded and launched Aave V1 in 2020. It has been more aggressive with features: flash loans (uncollateralized loans that must be repaid within a single transaction), credit delegations (allowing trusted parties to extend credit on behalf of others), and cross-chain deployments. Aave V3 introduced portal functionality that enables capital to move between different blockchain networks. As of early 2025, Aave consistently leads in total value locked, often holding $10 billion or more across its deployments.

Other protocols exist—Compound’s governance token holders have forked into projects like Goldfinch (which uses off-chain credit assessment) and Centrifuge (which tokenizes real-world assets). But for understanding the core mechanics, Aave and Compound are the essential references.

Risks You Need to Understand

DeFi lending isn’t free money. The risks are real and sometimes invisible until you’re caught.

Smart contract risk is the first concern. Lending protocols are code, and code can have bugs. While Aave and Compound have undergone multiple security audits and survived years without major exploits, the history of DeFi includes numerous protocols that lost user funds to hacks. Even audited contracts can contain vulnerabilities that manifest under unusual conditions.

Liquidation risk applies to every borrower. If you’re borrowing against collateral, market volatility can trigger liquidation faster than you can react. During the May 2022 collapse of Terra’s UST stablecoin, many borrowers were liquidated instantly as cascading selling drove asset prices off cliffs. The lesson: don’t borrow close to your collateral limits.

Supply risk means your deposited assets can lose value if the borrowed asset’s price drops relative to your collateral. But more fundamentally, the tokens you receive (aTokens, cTokens) are exposure to the protocol itself. If the protocol were compromised, your assets could be lost.

Finally, there’s regulatory uncertainty. DeFi protocols operate without KYC, but regulators worldwide are still deciding how to treat these systems. Future rules could impact how you access or use lending protocols.

Getting Started: A Practical Path

If you want to participate in DeFi lending, the process involves wallet setup, protocol interaction, and position management. Start small.

You’ll need a Web3 wallet like MetaMask or Rabby, funded with cryptocurrency (ETH for gas fees, plus the asset you want to supply). Navigate to the protocol interface—Aave’s app at app.aave.com or Compound’s at compound.finance. Connect your wallet, select the asset you want to deposit, approve the token, and confirm your transaction.

Once your deposit is confirmed, you can borrow against it if desired. Start with modest borrow amounts—keeping your health factor above 2.0 gives you a comfortable buffer. Monitor your position regularly, especially during volatile periods.

Before depositing significant amounts, consider the asset’s risk rating on the protocol. Each asset has different collateral factors, liquidity, and historical volatility. New or unproven tokens carry higher risk of sudden price crashes that could trigger liquidation.

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DeFi lending protocols represent one of the most functional applications of blockchain technology. They enable anyone with cryptocurrency to earn yield or access liquidity without permission from any institution. The mechanics—pooled lending, algorithmic interest rates, overcollateralized borrowing, automated liquidation—work together as a self-regulating financial system.

What remains genuinely unresolved is how regulators will treat these protocols. Traditional finance has never operated without intermediaries, and governments are still figuring out whether decentralized code constitutes a financial service requiring licensing. The next few years will determine whether DeFi lending grows within a regulated framework or continues in its current gray zone. The protocols themselves are functioning flawlessly; the uncertainty lies entirely in the human systems surrounding them.