This collection marks the 20th anniversary of the entry into force of the UN Optional Protocol to the Convention Against Torture (OPCAT). It focusses on one major component of  a  growing network of monitoring bodies, that is, the establishment of NPMs (National Preventive Mechanisms) in a European context.  The contributors come from different parts of Europe and bring their unique expertise into the book.  In their chapters they reflect on questions related to their countries' processes in ratifying OPCAT as well as the establishment and functioning of the NPMs.  The book explores the variety of ways in which NPMs implement their mandate in practice, as well as the key opportunities and challenges they encounter in fulfilling their role.