The term 'transaction' is frequently used in discussions of economic activity, but is rarely the direct subject of examination, despite its apparent centrality. This is true whether its role is ascribed to the material mediation of money, to financial capital more generally, or to the rationalized technical activity at the core of large formal organizations. The modern representation of a transaction, however, began as a practical solution to a very specific technical question: how can a computer system successfully handle simultaneous requests contending to rapidly read and write from the same large data resource?,This dissertation makes the case for the sociotechnical formalization of the transaction, along with that of message-oriented data communications, as fundamental prerequisites and facilitators for significant transformations in finance, including the development of electronic securities exchanges. However, these transformations were as political as they were technological. Through an examination of competition and regulation in the securities exchange industry, I show how markets—both in practice and in theory—are redefined by the prevalence and interrelation of reliable distributed platforms for automatically matching buyers and sellers, executing transactions, and broadcasting quotes and trade reports. The resultant redefinition of the exchange as a sociotechnical ‘platform’ then provides a novel framework for the analysis and regulation of contemporary marketplace platforms.