This paper presents a model of repo rehypothecation in which dealers intermediate funds and collateral between cash lenders (e.g., money market funds) and prime brokerage clients (e.g., hedge funds). Dealers take advantage of their position as intermediaries, setting different repo terms with each counterparty. In particular, the difference in haircuts represents a positive cash balance for the dealer that can be an important source of liquidity. The model shows that dealers with higher default risk are more exposed to runs by collateral providers than to runs by cash lenders, who are completely insulated from a dealer's default. In addition, collateral providers' repo terms are sensitive to changes in a dealer's default probability and its correlation with the collateral's outcome, whereas cash lenders' repo terms are unaffected by these changes. This paper rationalizes the difference in haircuts observed in bilateral and tri-party repo markets, reconciles the partial evidence of the run on repo during the recent financial crisis, and presents new empirical evidence to support the model's main prediction on haircut sensitivities.