Central banks play a pivotal role in well-functioning payments systems by providing liquidity via collateralized lending. This article discusses the role of collateral and haircut policy in central bank lending, as well as the distinguishing features of the central bank’s policy relative to private sector practices. It presents a model that explicitly incorporates the unique role of central banks in the payments system and argues that central banks must consider how their haircut policies affect the relative price and liquidity of assets, the market’s asset allocation, and the likelihood of participants to default. Furthermore, under extraordinary circumstances, there is a rationale for the central bank to temporarily reduce haircuts or broaden the list of eligible collateral to mitigate the shortage of liquidity in the market.