The demand for comparable, long-term, high quality data on forest ecosystems' status and changes is increasing at the international and global level. Yet, sources for such data are limited and in many case it is not possible to compare data from different monitoring initiatives across space and time because of methodological differences. Apart from technical manuals, there is no comprehensive multidisciplinary, scientific, peer-reviewed reference for forest monitoring methods that can serve and support the user community. This book provides in a single reference the state-of-the-art of monitoring methods as applied at the international level. The book present scientific concepts and methods that form the basis of the transnational, long-term forest monitoring in Europe and looks at other initiatives at the global level. Standardized methods that have been developed over two decades in international forest monitoring projects are presented. Emphasis is put on trans-nationally harmonized methods, related data quality issues, current achievements and on remaining open questions. It offers a comprehensive overview of needs, requirements, organization and possible outcomes of an integrated monitoring program. It features tested and quality assured, internationally harmonized methodologies based on a complete revision of existing methods carried out in 2009-2011. It is connection with monitoring results allows assessment of the potential of the monitoring method.