Angular momentum is one of the most fundamental properties of matter in our universe, which has deep consequences on its evolution. In particular, the formation and the physical characteristics of cosmic structures such as galaxies, stars and planets are intimately linked to the amount of angular momentum they carry, to the way it is redistributed within the system and exchanged with the surrounding environment. Considerable efforts have been undertaken during the last decades to identify and quantify the various physical mechanisms responsible for the transport of angular momentum in these objects. While some of them are relatively well understood, in many circumstances the underlying mechanism turns out to be extremely complex and very challenging. In this introductory chapter, we first introduce some general considerations on the angular momentum impact and transport. We then derive the MHD equations both in the ideal and non-ideal limit. Finally, after deriving the conservative form of the angular momentum equation, we discuss in more details some of the mechanisms that can contribute to the transport of angular momentum in various astrophysical contexts.