This thesis deals with investment arbitration, a type of international arbitration. The basis of investment arbitration is a multilateral or bilateral investment treaty, the national investment law of the Host State or an investment agreement. Investment arbitration involves public international rather than private law and treaties in addition to or instead of contracts. Furthermore, disputes in international investment law can arise between the contracting parties or between the investor and the Host State, and BITs address both issues. Contracting states most often opt for rules of ICSID or the UNCITRAL Rules. Moreover, because of the New York Convention and the ICSID Convention, foreign investors can claim against Host States without any form of governmental screening. Most arbitrations concerning the EU Member States arise out of BITs (bilateral investment treaties) under particular provisions on investor-State arbitration. Also, most investor-State arbitration procedures are based on an agreement to arbitrate which is BIT and plurilateral agreement. In addition, investor-State arbitration allows a foreign investor from 'Home State' to file a claim against a 'Host State', or a State in which investor has made an investment. For investment arbitration, the most important thing is to define terms ‘investor’ and ‘investment’. The right of an investor to call upon protection under the treaty depends on his nationality. With respect to the shareholding, even if a company does not meet the criteria for the nationality qualification, the foreign investor is allowed to pursue the claim in his own name, which is also applicable for minority shareholdings. Most of the BITs authorize foreign investor to file damages claims directly against Host States before international tribunals. Bilateral investment agreement can be made between two states or between investor and the Host State. Since the adoption of the Lisbon Treaty, the European Commission has expressed concerns with regards to the validity of the intra-EU BITs and stated that these BITs should be terminated. On 13 September 2017 the European Commission adopted the Recommendation for establishment of the multilateral court for settlement of international disputes. Finally, there are many concerns regarding investment arbitration. However, the European Federation for Investment Law and Arbitration, and many others, show that investment arbitration, indeed, is a successful tool for investment dispute settlement.