Admissibility of illegally obtained evidence is one of the procedural issues in the investment treaty arbitration, on which the tribunals do not have the unified approach or point of view. Although, there are already cases where the tribunal have been faced with this problem, and this gives the ground for the thoughts about the topic. Provided the growing challenge for the tribunals on whether to admit or reject such evidence, this topic remains actual in the field of the arbitration.  The aim of the thesis is to highlight the legal issues, related to the admissibility of illegally obtained evidence in investment treaty arbitration. The thesis analyses three sub-issues: is illegally obtained evidence admissible in investment treaty arbitration, if it is admissible under which conditions and which types of illegally obtained evidence exist currently.