Hundreds of thousands of participants make tens of thousands of trades every day for thousands of virtual goods in a large video game community; weekly volume has a US dollar equivalent of about $2 million.  We have access to every transaction in the entire economy over a span of 1.8 years, yielding a uniquely detailed and complete picture.

The computer platform was designed to make barter exchange as attractive as possible; money was not part of the design and all players were created equal. Yet, within weeks, several specific goods began to emerge as media of exchange, as did specialized traders. By the end of the period the vast majority of trade (in value terms) was money-mediated and/or handled by specialists.

In this paper we use classic economic concepts together with new network theoretic tools to document the evolution of money and other market institutions.