Soft law plays an important role in the regulation of international tax matters. This paper focuses on the case of the OECD Transfer Pricing Guidelines and analyses the relationship between this non-binding instrument and the formal sources of law. From the perspective of international law, the OECD Guidelines are mainly connected to double tax treaties, but they may also influence customary norms and the general principles of law. From the point of view of domestic legal systems, references to the OECD Guidelines can be found in the tax legislation of some countries and, especially, in the interpretative circulars of the Tax Administrations. Furthermore, in some states the courts have also taken the OECD Guidelines into account in their judgements, which shows that they are almost treated as hard law. However, this practical relevance of the Guidelines does not seem to be in accordance with the process in which they were made in the OECD, which could be more open to the different stakeholders and more transparent.