Since 11 September 2001 the struggle against international terrorism has been one of the major commitments of governments worldwide. The attacks in Madrid and London in 2004–05 made it a particular political priority in Europe, with the European Union (EU) and its Member States putting surveillance measures, mainly through data monitoring, collection and retention, at the heart of their counterterrorism strategies. The impact of these tools on personal freedoms and the responses by European supranational and domestic courts are examined in this important book.

Data retention is an unorthodox surveillance measure, but an important one. It does not concern the content of communications but information about the communication (metadata), the cross-referencing of which can reveal important details concerning a person's habits, preferences and thoughts, facilitating the profiling of individuals. As a result, data retention can seriously interfere with privacy and rights of data protection, engaging the delicate balance between rights and freedoms, on the one hand, and security, on the other, which in turn impacts the very foundations of democracy.

Moreover, data retention—and surveillance in general—engages a third ‘actor’ in the rights–security relationship, this being technology. Technology is a double-edged sword: whilst it can be used against terrorists, it facilitates activities which can seriously endanger rights and freedoms.

This volume surveys the case law of selected supranational courts and domestic supreme and constitutional courts concerning data retention. Its purpose is to identify some common standards that emerge from the vertical and horizontal judicial dialogue which has taken place on this subject. From this angle, it might have been helpful to enlarge the perspective to include decisions of other than apex courts (supreme and constitutional), which have a pivotal role in judicial dialogue. Among others, in the United Kingdom—when it was still part of the EU—some key rulings on data retention were delivered by lower courts, so addressing them might have been of assistance.

The book is divided into three parts, preceded by a brief Editorial Introduction which usefully sets the scene.

Part I comprises two chapters, concerning data retention in the EU and in the case law of the European Court of Human Rights (ECtHR) respectively. They comprehensively explore all EU law matters—including the data retention directive, its invalidation by the ECJ and the development of its case law—along with its engagement with the rulings of the ECtHR.

Part II consists of 12 chapters concerning data retention in selected EU Member States whose supreme and constitutional courts have reacted, in some way, to landmark supranational judgments concerning this. These chapters helpfully follow a common template, making their content easier to follow.

After setting out the domestic legislation implementing the 2006 EU data retention directive in each State, the authors consider whether such national measures had already been the subject of any constitutional or supreme court decisions before the European Court of Justice (ECJ) ruled the data retention directive invalid in 2014. The domestic response to that ruling is then examined.

These chapters are also of great value as they shed light on the regulatory regimes of a number of States—such as Bulgaria, Czech Republic, Poland, Romania, Slovakia, Slovenia—whose approach has rarely been addressed by scholarly works in English.

There is also a very useful annex of extracts, translated into English, of all judgments referred to in the chapters. This further helps disseminate knowledge of the data retention laws and standards of a wider range of domestic contexts than is found in other works.

Part III is the heart of the book, analysing and taking stock of the rulings examined in previous chapters in order to identify common standards within the case law. These standards are not merely described, but are also compared and their key elements highlighted, which greatly enhances the overall significance of the book. Whilst some of these standards may become outdated in the light of more recent judgments of the ECJ, the ECtHR and some domestic courts, others retain their more general significance. For instance, neither the ECJ nor domestic constitutional courts seek to place an absolute ban on data retention and mass surveillance more generally.

Rather, the courts highlight the importance of such measures respecting the proportionality principle and, consequently, on the need to limit these measures to the prevention and prosecution of very serious offences, such as terrorism. This has recently been stressed by the French Council of State in its La Quadrature du Net decision (2021), and by the ECtHR in Big Brother Watch (2021). Another example concerns the role of private entities in the data retention process. It is noted that whilst many constitutional and supreme courts have accepted that there may be interplay between private bodies and public authorities in security-related operations, this must be time-limited and subject to precise rules, a stance similar to that embraced by the ECJ in the recent Privacy International case. These examples, and others, illustrate the extent of the judicial dialogue which is now occurring in Europe concerning data retention issues.

All chapters are written in a clear and accessible style, but without ignoring technical details. This combination of clarity and technicality makes the book valuable not only for scholars and practitioners (lawyers and policymakers) with expertise in these matters, but also for those approaching the topic for the first time. This volume is, therefore, a useful tool for a wide audience.

In conclusion, there is no doubt that this book provides a valuable overview of the evolution of European surveillance law (and related case law) in the first two decades after the 9/11 attacks. One must hope that European supranational and domestic courts will continue to follow the trends highlighted in this book and that lawmakers will adopt legal frameworks which are in compliance with the judicial standards that this book identifies. This is more necessary than ever in a world where, due to ‘global’ emergencies, such as the COVID-19 pandemic, uncertainty and insecurity are fostering limitations on rights and freedoms which are progressively becoming ‘socially approved’, as the editors put it. Social acceptance, however, should not result in departures from the rule of law, which is where courts should continue to step in.