This chapter looks at triangulation, which is classically defined as looking at one research object from different perspectives. However, this large and consensual definition masks different approaches to triangulation and ignores its historical evolution since its emergence in social sciences literature. To gain a better insight into its current definitions, the chapter first proposes a brief historical overview and highlight its different meanings. It then illustrates how triangulation can be used in a research design in order to gain extra knowledge. Finally, the chapter talks about mixed-methods research and its relationship with triangulation. In the context of the tensions opposing qualitative and quantitative research, triangulation is used by mixed-methods research to justify that qualitative and quantitative methods should systematically be articulated.