In this article, the author analyses the “arm’s length principle” and the “associated enterprise” concept, both from a historical perspective, as well as in the context of the proposed EU Transfer Pricing Directive. The article begins with a comparative analysis of the OECD practice regarding the ALP and that of Belgium. It then looks at the concept of “associated enterprise” under article 9 of the OECD Model (2017) and the proposed definition of the term under the TP Directive. The author then delves deeper into the potential legal issues that could arise if those definitions end up being distinct from international legal standards, focusing notably on EU companies operating within and outside the Single Market and proposing solutions to improve the impact on the Single Market.