A global shortage of organs for transplantation has led to myriad proposals for improving supply. One that has gained significant traction is so-called “opt out”, wherein donation becomes the default, and the burden falls on those who do not want their organs used to formally record this. Despite widespread uptake, opt out models are far from flawless. They have, in their theoretical form, been criticised for masquerading as informed consent and inadequately involving families in decisions. In their realised form, they have been suggested to be redundant given practical challenges in following through with the core tenet. Alongside such critiques are concerns around designing opt out systems to satisfy public awareness requirements and protect the vulnerable from inappropriate inclusion. This book offers the first comprehensive analysis of all four UK opt out systems. Rather than assessing the success of these systems in increasing transplantation activity - which it is ultimately too soon for - this book examines the paths to and design of these systems. In dissecting the construction, it manoeuvres towards a better understanding of what a “good” opt out system might look like, offering important considerations in the development of future examples elsewhere. Through this analysis, this book also explores the relationship between the four UK nations in their respective moves to introduce opt out. Further, it incorporates comparative discussion with the systems of the Crown Dependencies, highlighting important interplays between the UK and these neighbouring jurisdictions. In doing so, it contributes to the growth of scholarly interest in the impact of devolution on healthcare law.