The book provides analysis of the principal rules of trust law which control the exercise of powers and discretions by trustees. The primary focus is on the principle known as "the Rule in Re Hastings-Bass", and this is considered alongside the doctrines of fraud on a power and mistake. This is the first book-length treatment focussed on this specific aspect of trust law, and in particular the first on the Rule in Re Hastings-Bass, which is the subject of much professional and academic interest especially following consideration by the Supreme Court in Pitt v Holt and Futter v Futter [2013] UKSC 26. Whilst considering Pitt and the Rule in Re Hastings-Bass alongside mistake and fraud on a power, the book also explains how these doctrines interact, and how the law regulates trustee decision-making as a whole. It sets out examples and considers extensive practical problems, allowing the reader to understand not only the core trust law rules, but also the broader consequences of those rules which arise in real cases. This aspect of trust law is of great practical importance for practitioners as it arises frequently in the context of trust litigation, and in advising trustees and beneficiaries of their rights and obligations. The newly settled state of the law after Pitt will encourage reliance on the Re Hastings-Bass and mistake rules by practitioners in challenges to trustees' conduct and decisions. This book equips all involved with the key principles and arguments in this area.