Knowledge fusion aims to integrate different knowledge graphs into a unified, consistent, and concise format, thereby establishing interoperability for applications that leverage multi-source knowledge. In this chapter, we introduce the research process of knowledge fusion. We begin with an exploration of the fundamental principles of multi-source knowledge fusion, followed by a survey of the technical advancements in knowledge fusion. Subsequently, we outline potential future research directions and conclude with a summary. We believe that the combination and mutual promotion of knowledge fusion and other knowledge graph technologies will make a big difference.