We study a dynamic timing game between multiple firms, who decide when to go public in the presence of possible information externalities. A firm's IPO pricing is a function of its privately observed idiosyncratic type and the level of investor sentiment, which follows a stochastic, mean-reverting process. Firms may wish to delay their IPOs in order to observe the market reception of the offerings of their peers. We characterize the unique symmetric threshold equilibrium, whereby pioneer firms with high idiosyncratic types endogenously emerge. The results provide novel implications regarding variation in IPO timing, sequential clustering, IPO droughts, the composition of new issues over time, and how IPO volume fluctuates over time. These include, among others, that in more populated industries, a lower proportion of firms emerge as industry pioneers, but follower IPO volume is intensified. Additionally, heightened uncertainty over investor sentiment exacerbates delay and leads to lower IPO volume.