I develop a dynamic model of collateral circulation in a repo market, where a continuum of institutions borrow from and lend to one another against illiquid collateral. The model emphasizes an important tradeoff. On one hand, easier collateral circulation makes repos liquid and increases steady state investment through several multiplier effects, improving economic efficiency. On the other hand, it can harm financial stability because less capital is sitting on the sidelines waiting for

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... nt opportunities. This fragility is further exacerbated by the endogenous repo spread through a positive feedback loop, and can result in an inefficient repo run. The model is relevant for understanding the repo markets during the financial crisis of 2008.