This chapter sets out to reconstruct Weber's account of how norms are enforced, and trust maintained, in the London Stock Exchange of his times, which he considered exemplary for its organizational effectiveness and incorporation of the capitalist ethos. It provides a brief description of the sociological literature concerning financial markets in general, with reference to their structure and ability to control improper behaviors, called "opportunistic" in the language of rational choice theory. The chapter considers how rational choice theory and Parsonian functionalism have accounted for the existence of social norms and trust, and the extent to which these theoretical perspectives are compatible with Weber's notions of rational action and social norms. It describes Weber's account will be discussed against the background provided by today's structural and cultural perspectives bearing on financial markets, with particular reference to the Berlin and London Stock Exchanges.