Explains how to identify and capitalize on marketplace design opportunities. Defines markets and marketplaces and describes the basic functions of each. Discusses attributes (e.g., heterogeneity of participants' preferences and asymmetry in available information) that determine whether and how marketplaces create value. Explains common causes of market failure and presents a framework for designing marketplaces in response. Discusses tactics for building trust and liquidity when launching new marketplaces as well as challenges encountered as marketplaces mature (e.g., congestion and disintermediation).