When-issued trading is the trading of securities prior to the actual issue of the security. When-issued trading is active around the world and in a variety of equity and bond markets. In this survey, we provide a general description of when-issued trading, analyze benefits and costs in various financial markets, present existing theoretical models and predictions, and synthesize empirical findings. We find that when-issued trading promotes price discovery, mitigates information asymmetry, provides convenience for trading ahead of the actual issue of the security, and in some markets reduces volatility. In addition, we offer policy implications and suggest directions for further research in this area.