Robert Lamersdorf examines fiduciary relationships-relationships in which one party has both the power and the duty to pursue and safeguard the other party's interests to the best of its discretion. Fiduciary relationships are of great intrinsic and instrumental value. Yet the power held by a fiduciary entails manifold opportunities for abuse. From the perspective of comparative law and with reference to U.S. fiduciary law, the author explores what legal safeguards are required, in view of the risk of abuse, to enable a productive fiduciary cooperation. In doing so, he focuses in particular on the limits of what the law can achieve directly. One core thesis is that coercion, liability, and sanctions create indispensable counter-incentives to the abuse of power, but can secure trust and loyalty only to a limited extent. Effective cooperation in a fiduciary relationship instead depends to a significant degree on an extralegal normative infrastructure that can be described, in summary, as a "culture of trust." This includes internalized social and moral norms, social roles, habits, dispositions, and basic assumptions. Robert Lamersdorf shows that this constitutes a form of social capital that bears the characteristics of a public good and is therefore threatened by underinvestment and negative externalities. Building on this, he develops a further thesis: fiduciary law has a regulatory function, namely to protect and preserve this social capital and thus the public good of "trust in fiduciaries."