This paper presents a model in which requiring banks to issue a proper amount of subordinated debt can constrain their risk taking both before and after debt issuance. The main idea is that the prospect of issuing debt motivates banks to invest in safe assets before debt issuance; holding such assets then constrains their risk taking after debt issuance. The model helps understand the existing empirical findings, and offers a new testable prediction. It also suggests that: (1) regulators should set the amount of subordinated debt within a range; and (2) subordinated debt cannot entirely substitute for equity capital.