This chapter deals with different phenomena traditionally labeled as phraseological variation. With the help of theoretical contributions from the field of phraseology, as well as from usage-based and diachronic linguistics, it provides tools to better understand this phenomenon. As a first step, the author will provide a brief introduction of the importance of variation in the frame of phraseological theory. Secondly, she will propose a definition for her study object, PUs, seen as a lexical units. Subsequently, she will focus on the diachronic process resulting in the emergence of a PU: phraseologization. This evolution process will give the keys to the understanding of variation in phraseology.