Private equity (PE) investments are investments in privately-held companies, that trade directly between investors instead of via organized exchanges. PE is often considered a distinct asset class, and it differs from investments in public equity in fundamental ways. There is no active market for PE positions, making these investments illiquid and difficult to value. PE funds typically have horizons of 10-13 years, during which the invested capital cannot be redeemed.We survey the academic research concerning the risks and returns of institutional PE investments, as well as the optimal holdings of PE in an investment portfolio. Researchers have had limited access to information about the nature and performance of PE investments, and so research in this area is still preliminary and often inconclusive.