This chapter focuses on the relationship of trustees to non-trustees who hold some power under the terms of the trust. The chapter examines the relationship from the trustees’ point of view, and considers how trustees can best ensure they do not commit a breach of trust in relation to the exercise of such a power by a third party. First, the chapter outlines some typical forms of third-party power. While third-party powers are bespoke, each the product of an individual trust instrument, it is possible to discern broad groups of common third-party powers. Secondly, the chapter considers what may likely go wrong in connection with each type of power, and considers the liabilities of the trustees which may arise in consequence. Understanding these liabilities demands a clear and accurate understanding of some very basic principles of trust law and equity. The chapter also considers how trustees should act in the light of these potential liabilities. Next, the chapter considers what drafting techniques may be used to make life easier for trustees who have to deal with a third-party power. Finally, it takes a brief look at the possibility of legislation in this area.