Synthesis of different types of knowledge is an essential part of all sciences, especially those that build on heterogeneous contributions from many scientific disciplines and thematic fields (e.g., environmental sciences). Synthesis takes place within and across disciplines and fields (interdisciplinarity), as well as between disciplines, fields and sectors (transdisciplinarity). Counterbalancing overspecialization in science, synthesis serves different purposes. Bearing such purposes in mind, this entry defines synthesis as an intentional and purposeful collaborative process that generates new knowledge by integrating heterogeneous contributions from different disciplines, fields and sectors (e.g., questions, concepts, theories, methodologies, results, and data) across spatial or temporal scales to establish previously unrecognized connections between them. It also defines different synthesis stages, methods and procedures and discusses some of the challenges inherent in such collaborative processes.