This chapter examines the fiduciary principles governing investment advice. Fiduciary principles in investment advice are both straightforward and complex. They are straightforward because most investment advisers are considered fiduciaries and subject to strict fiduciary duties under federal and state law. Their complex nature arises from the fact that many individuals and firms provide investment advice but are not deemed investment advisers and, therefore, are not subject to a fiduciary obligation. This chapter first explains whether and when an advisory relationship gives rise to fiduciary duties by focusing on both federal and state law, as well as the individuals and firms that typically provide investment advice. In particular, it looks at certain persons and entities excluded from the definition of investment adviser and thus not subject to the Investment Advisers Act of 1940, namely broker-dealers, banks, and family offices as well as accountants, lawyers, teachers, and engineers. The chapter also considers fiduciaries under ERISA, the Investment Company Act, and the Commodity Exchange Act before discussing the fiduciary duty of loyalty and how it is expressed and applied in investment advisory relationships; the fiduciary duty of care and how it differs from other standards of conduct, such as a duty of suitability; and other legal obligations imposed on investment advisers and how those obligations relate to an adviser’s fiduciary duty. Finally, the mandatory or default terms with regard to an investment adviser’s fiduciary duties are explored, along with remedies available for breach of fiduciary duty.