This book addresses the effectiveness of existing protected areas at conserving the diversity of genes, species and ecosystems on land and at sea. The book synthetizes the main biodiversity conservation outcomes of protected areas in the 2010-2019 decade, drawing on a systematic literature review of scientific publications and case studies from around the world demonstrating successes and failures. It provides region-specific results for land and sea ecosystems as well as for developed and developing countries. It also reviews current methodological approaches used to assess protected area effectiveness. The work is timely, since there is growing concern on the global biodiversity crisis among researchers, government organizations and the general public, as demonstrated by the 2030 targets established by the UN Sustainable Development Goals for Life Below Water (SDG 14) and Life On land (SDG15). The book is written in an easy and enjoyable style using numerous pictures, tables and graphs to make the content more engaging and understandable. The main intended audiences of the book are academics, from post-graduate students to university lecturers, and senior researchers in the fields of biodiversity conservation, sustainable development and environmental policy, as well as protected area managers and practitioners.