Transdisciplinary research aims to generate knowledge through science-practice interactions and is attracting increasing attention in sustainability science. However,sustainable development is characterized by vast complexities and context specificities, with a broad spectrum of relevant actors. Transdisciplinary research projects targeting at socio-ecological transformations, therefore, require the integration of different bodies of knowledge from diverse actors to gain an accurate and comprehensive understanding of existing problems and potential solutions. This knowledge integration involves both the active exchange of knowledge and the creation of a joint understanding of problems and solutions. Hence, the focus of this article is to analyse how and why knowledge integration is successful in transdisciplinary research processes. We conceptualize and define knowledge types and goals of knowledge integration. By analysing seven transdisciplinary research projects, we develop and empirically test indicators for knowledge integration. We examine knowledge integration (i) within three specific knowledge types (systems, target, and transformative knowledge), and (ii) between these different knowledge types. Furthermore, we reflect on the roles of researchers and non-academic actors. The description, comparison and discussion of knowledge integration lead to the identification of barriers and success factors to improve such processes in transdisciplinary research activities.