In this article, we empirically document how primary dealers use and re-use collateral in the United States. Using confidential supervisory data, we precisely map the flow of collateral to and from individual dealers and identify whether the collateral used in those transactions is encumbered or rehypothecated. From these data, we can characterize how different cash and secured financing transactions affect dealers' balance sheets and present some stylized facts of their operations. We present three measures of collateral use and re-use at the dealer level to proxy for the amount of collateral circulation in the U.S. financial system. We find evidence from one of our measures that the degree of collateral circulation is significantly higher for U.S. Treasury securities and highlight the special role repurchase agreements (repos) play in their intermediation. Characterizing dealers' use and re-use of collateral contributes to ongoing research aimed at understanding how collateral circulation improves market functioning by increasing the availability of collateral but may also lead to financial fragility by increasing the amount of interconnectedness in the financial system.