The functioning of sovereign bond markets is closely intertwined with repo markets, where investors finance long positions and source bonds for short sales. We show, theoretically and empirically, how conditions in the cash bond market fundamentally shape repo rates: When demand for cash bonds exceeds available supply, arbitrageurs, notably hedge funds, step in to bridge the gap by shorting bonds and borrowing them in the repo market. These arbitrage strategies drive a wedge between monetary policy rates and repo rates, generating “specialness”. To investigate the drivers of investors’ demand for repo contracts, we obtain regulatory data covering the entire universe of repos backed by German sovereign bonds. We identify the ultimate borrowers and lenders of securities and estimate their demand and supply elasticities for the repo market. Supply, dominated by the public sector, accounts for 87% of the aggregate elasticity. Demand, driven mainly by hedge funds, is strongly inelastic: a 10% increase in borrowing costs reduces their borrowing by only 1% […]