Investing in illiquid assets has become increasingly popular among individual and institutional investors. Illiquid assets provide higher returns and interesting diversification alternatives for asset managers.

To the contrary of traditional assets that investors are familiar with, the alternative investments require deep knowledge of their intrinsic properties. This knowledge is necessary to value the underlying and determine their share in portfolios. As the last financial crisis showed, misgauging the implications of illiquidity can have severe consequences. The current article surveys the existing literature that enhances our understanding of illiquid assets, their attractiveness, their valuation, and most importantly, their optimal allocation in investment portfolios.