Proportionality is a German, and thus continental European, concept in public law that is applied by both the Court of Justice of the European Union (CJEU) and the European Court of Human Rights (ECtHR). The principle specifies that measures adopted by executive authorities should not exceed the limits of what is appropriate and necessary in order to achieve legitimate objectives in the interest of the public. Using a functional comparative approach, this book evaluates the extent to which proportionality has been integrated into the English and Hong Kong judicial systems by comparing case law in these courts with that of the CJEU and the ECtHR. The text also reviews the development of proportionality and presents a topical understanding of why its adoption and application have encountered difficulties, particularly regarding socio-economic rights, in some jurisdictions, such as the United Kingdom and Hong Kong. Written by a scholar with experience from both within the Hong Kong judicial system and from international research, this book is the first all-encompassing reference for legal practitioners worldwide. More