International investment arbitration involves high costs both for the investor and the State appearing in proceedings. A relatively recent phenomenon in international investment arbitration is the financing of the proceedings by a third party funder. The third party funder as such has no interest in the substantive issues of the arbitral proceedings, but instead invests in the proceedings hoping to make a considerable profit upon the settlement of the dispute. International arbitrators have in principle no competence to address the third party funding agreement because their competence is limited to the dispute between the foreign investor and the host State. The funding agreement is thus alien to the legal relations between the foreign investor and the host State. After an analysis of the rationale, concept and principles of third party funding, this Article will analyse whether tribunals may nevertheless use their discretion to intervene in or take into consideration the relationship between the investor and its third party funder, in particular in view of the allocation of costs in the arbitral proceedings. This Article will also investigate whether the existence of a funding agreement is subjected to any rule on transparency and to an obligation of disclosure, in essence in order to ensure the respect of the principle of ‘equality of arms’.