This chapter restates the economic theory of fiduciary law, making several fresh contributions. First, it elaborates on earlier work by clarifying the agency problem that is at the core of all fiduciary relationships. In consequence of this common economic structure, there is a common doctrinal structure that cuts across the application of fiduciary principles in different contexts. However, within this common structure, the particulars of fiduciary obligation vary in accordance with the particulars of the agency problem in the fiduciary relationship at issue. This point explains the purported elusiveness of fiduciary doctrine. It also explains why courts apply fiduciary law both categorically, such as to trustees and (legal) agents, as well as ad hoc to relationships involving a position of trust and confidence that gives rise to an agency problem.Second, this chapter identifies a functional distinction between primary and subsidiary fiduciary rules. In all fiduciary relationships we find general duties of loyalty and care, typically phrased as standards, which proscribe conflicts of interest and prescribe an objective standard of care. But we also find specific subsidiary fiduciary duties, often phrased as rules, that elaborate on the application of loyalty and care to commonly recurring circumstances in the particular form of fiduciary relationship. Together, the general primary duties of loyalty and care and the specific subsidiary rules provide for governance by a mix of rules and standards that offers the benefits of both while mitigating their respective weaknesses. Finally, this chapter revisits the puzzle of why fiduciary law includes mandatory rules that cannot be waived in a relationship deemed fiduciary. Committed economic contractarians, such as Easterbrook and Fischel, have had difficulty in explaining why the parties to a fiduciary relationship do not have complete freedom of contract. The answer is that the mandatory core of fiduciary law serves a cautionary and protective function within the fiduciary relationship as well as an external categorization function that clarifies rights for third parties. The existence of a mandatory fiduciary core is thus reconcilable with an economic theory of fiduciary law.