Legal scholars highlight the tensions that exist between different classes of shareholders in startups. We model a startup owned by undiversified investors with heterogeneous capital contributions and risk preferences. A social planner runs the firm on behalf of all investors. We compare investors’ expected utility with a hypothetical first-best decentralized benchmark. The startup’s optimal investment policy is procyclical and a time-varying weighted average of shareholders’ optimal investment policies. The optimal contracts issued to investors are tailor-made, interdependent, and include equity claims resembling preferred stock with heterogeneous payout caps, leading to a complex capitalization table as more investors join the startup. This paper was accepted by Will Cong, finance. Funding: This work was supported by the Cambridge Endowment for Research in Finance and Keynes Fellowship. Supplemental Material: The online appendices and data files are available at https://doi.org/10.1287/mnsc.2022.01724 .