This chapter illustrates the possible meanings of transparency in the literature. It describes its origin in the legal domain, focusing on international law and EU law. Following an extensive literature review on the notion of transparency, the book highlights that it can be described as a “multidimensional” concept linked with several purposes. Due to the often overlapping meanings of transparency, the chosen approach serves to clarify its purposes by describing its related “dimensions”. “Democracy”, “legitimacy”, “accountability”, “realisation of individual rights”, and “market efficiency” are the transparency dimensions identified and chosen in this book. Each of these dimensions can help readers gain a deeper understanding of the transparency concept. This chapter also reflects upon the target of transparency obligations.