Abstract: This chapter explores the application of fiduciary duties to regulated financial firms and financial services. At first blush, the need for such a chapter might strike some as surprising in that fiduciary duties and systems of financial regulation can be conceptualized as governing distinctive and non-overlapping spheres: Fiduciary duties police private activity through open-ended, judicially defined standards imposed on an ex post basis, whereas financial regulations set largely mandatory, ex ante obligations for regulated entities under supervisory systems established in legislation and implemented through expert administrative agencies. Yet, as we document in this chapter, fiduciary duties often do overlap with systems of financial regulation. In many regulatory contexts, fiduciary duties arise as a complement to, or sometimes substitute for, other mechanisms of financial regulation. Moreover, the interactions between fiduciary duties and systems of financial regulation generate a host of recurring and challenging interpretative issues. Our motivation in writing this chapter is to explore the reasons why fiduciary duties arise so frequently in the field of financial regulation, and then to provide a structured account of how the principles of fiduciary duties interact with the more rule-based legal requirements that characterize financial regulation. As grist for this undertaking we focus on a set of roughly two dozen judicial decisions and administrative rulings to illustrate our claims.