The exercise of discretion by the trustees of a testamentary trust is increasingly a source of litigation. Although some judgments have paved the way for more thought, many questions remain unanswered. This text aims to establish how Quebec law regulates/controls trustees in the exercise of their powers. More precisely, it aims to determine whether trustees must decide considering their own perception of what the testator would have decided himself, or whether they must decide on the basis of how a reasonable person would decide. To answer the question in the context where very few authors have specifically addressed the discretionary powers of trustees up to now, this text builds upon the meeting point between the modern theory of the patrimony and the theory of private powers. This text also aims to reconcile the testator’s freedom of testation (which may even be exercised unreasonably) and the duty of the trustees to act as a reasonable person.