Stablecoins are the most common crypto IOUs. For actively traded positions, exchange IOUs are essentially unavoidable. For exchange IOUs, regulatory oversight and insurance create the primary quality distinction. A bank deposit IOU in the US is backstopped by $250,000 FDIC insurance, mandatory capital requirements, and central bank lender-of-last-resort access. DeFi introduces a third tier: protocol IOUs that eliminate the single-custodian failure mode while introducing smart contract risk.