This chapter canvasses the fiduciary principles applicable to a trustee of a donative, irrevocable private trust. The focus is on American law. The chapter examines: (1) the trigger for finding a trust fiduciary relationship and the scope of that relationship; (2) the duty of loyalty; (3) the duty of prudence across the distribution, investment, custodial, and administrative functions of trusteeship; (4) other fiduciary duties in trust law, including the prominent duty of impartiality and the increasingly salient duty to give information to the beneficiaries; (5) the extent to which fiduciary principles in trust law are mandatory or may be waived by the settlor or by a beneficiary; and (6) the remedies available for a breach of duty by a trustee.