We provide a theory of Special Purpose Acquisition Companies, or SPACs. A sponsor raises financing for a new opportunity from a group of investors with differing ability to process information. We show that when all investors are rational, the sponsor prefers to issue straight equity. However, when sufficiently many investors are overconfident about their ability to process information, the sponsor prefers to issue units with redeemable shares and rights, because such investors overvalue the option to redeem shares. The model matches many empirical features, including the difference in returns for short-term and long-term investors and the overall underperformance of SPACs. We also evaluate the impact of policy interventions, such as greater mandatory disclosure and transparency, limiting investor access, and restricting the rights offered.