This doctoral thesis considers the family’s role in the process of making medical treatment decisions for one of its members. Chapter 1 explores the meaning of family in the context of illness and disability. It is suggested that membership is not defined by biological or marital status but by interdependence, which also provides the moral and logical claim for a role of influence in treatment decisions. Chapter 2 considers various ethical frameworks for development of an inclusive decision-making framework, using fictional case studies. Chapter 3 examines the historical development of the legal framework for the resolution of treatment disputes revealing the ‘invention’ of a common law jurisdiction in respect of adults lacking capacity in order to meet perceived social need. Chapter 4 examines the unifying concept of the various jurisdictions in respect of adults and children, namely the patient’s ‘best interests’. Decided cases are categorised by their principal issue to demonstrate that the judicial concept of best interests has been shaped by core values intrinsic to those issues, leaving consideration of the interdependence inherent in the family context side-lined. That such disregard is not a necessary corollary of the mechanisms adopted for decision-making is shown by comparison with later cases of paternity testing disputes. Chapter 5 traces the last throes of the inherent jurisdiction through political engagement and the Bournewood litigation. Finally chapter 6 assesses the provisions of the Mental Capacity Act 2005. It is argued that, although the statutory test of ‘best interests’ is susceptible to familyfriendly considerations, the general authority limits involvement of families to trivial matters.