This thesis´ aim is to analyse the extension of the fuctionally separate entity approach  (FSEA) used in order to attribute a proper amount of profit to banks´ permanet establishment (PE) and the related implication in the authorised OECD approach (AOA). From a general perspective in fact, under the FSEA a PE is treated as being a separate entity from the head office, functions, assets, risks and funding shall be attributed as it was a separate legal entity. However, beyond the previous statement some general limitation of the FSEA and the particular nature of bank (and in general financial institutions) require a further reflection. Practically speaking: how far is the extension of this functional separate entity approach? Is it treatable as a complete separate company by itself? If not, how far is from the economic reality of the company? After a fisrt descriptive part, the author analysis the hybrid nature of the FSEA: acoording with his view traces of single entity approach are founded. However the hybrid nature of the FSEA is not per se a limitation but shall be considered as strength: the OECD´s approach grants a certain degree of flexibility and ability to adapt based on the context that on the other hand would have let the FSEA and the AOA be even more complicate.