Prison policy in Belgium and the Netherlands is changing rapidly. While Belgium struggles with a persisting prison overcrowding, the Netherlands strongly cuts back on the prison system and is closing an increasing number of prisons. This introductory article to a special issue on detention starts with a short outline of recent changes in Dutch and Belgian prison policy, focusing on developments in detention capacity and prison population. Subsequently we present an overview of empirical criminological research in the Netherlands and Belgium, situated within the international literature, with a specific focus on studies regarding life in detention and effects of detention on prisoners' lives and on their social environment. Finally, we reflect upon existing detention research in both countries, e.g. in terms of gaps in research topics and methodology, and discuss some future developments.