Abstract: This article develops an agency costs theory of the law of private trusts, focusing chiefly on donative trusts. The agency costs approach offers fresh insights into recurring problems in trust law including, among others, modification and termination, settlor standing, fiduciary litigation, trust-investment law and the duty of impartiality, trustee removal, the role of so-called trust protectors, and spendthrift trusts. The normative claim is that the law should minimize the agency costs inherent in locating managerial authority with the trustee and the residual claim with the beneficiaries, but only to the extent that doing so is consistent with the ex ante instructions of the settlor. Accordingly, the use of the private trust triggers a temporal agency problem (whether the trustee will remain loyal to the settlor's original wishes) in addition to the usual agency problem that arises when risk-bearing and management are separated (whether the trustee/manager will act in the best interests of the beneficiaries/residual claimants). The positive claim is that, at least with respect to traditional doctrines, the law conforms to the suggested normative approach. This Article draws on the economics of the principal-agent problem and the theory of the firm, and it engages the ongoing debate about whether trust law is closer to property law or contract law. Although the analysis focuses on donative trusts, it should be amenable to extension in future work to commercial and charitable trusts.