This book explores the relationship of mutual trust and fundamental rights in the Area of Freedom, Security and Justice (AFSJ) of the European Union and asks whether there is any role for proportionality. Mutual trust among Member States has for long been presumed by the Court in a manner that mutual recognition was prioritised vis-a-vis and to the detriment of the protection of fundamental rights. After thoroughly reviewing this relationship, the book offers a comprehensive framework of proportionality and explores its impact to the protection of fundamental rights in a mutual trust environment. It provides a theoretical and a normative framework of proportionality which then applies to two case-studies by reference to several fundamental rights. EU criminal and asylum law serve as the case studies of this book which enable a carefully constructed analysis with useful parallels. It argues that such analysis, based on proportionality, is not actually always desirable and helpful for the protection of fundamental rights in this area and thoroughly explores the impact of it on the protection of fundamental rights vis-a-vis mutual trust.