Ezequiel Heffes’ monograph entitled Detention by Non-State Armed Groups under International Law comes at the right time, and with its approach of merging a doctrinal legal analysis with case studies from the field, it fills an important gap in the literature. This book should be warmly welcomed by both academics and practitioners. We owe him gratitude for his thorough research and persuasive legal analysis, which pushes the frontiers of international law and opens new horizons.