In recent years, foreign investors have increasingly resorted to international arbitration against the Western Balkan States. In a number of such arbitrations, the respondent States have raised an illegality defence, arguing that the applicable treaties bar access to arbitration to investors who have obtained their investments illegally. This defence strategy has generally proven unsuccessful. This chapter aims to distill the lessons from publicly available awards in which the Western Balkan States have raised an illegality defence. It surveys the approaches taken by tribunals in those disputes and considers the types of illegalities that have been deemed to deprive an investor of access to arbitration, when such illegality must have occurred, and what State conduct may weaken the State’s illegality defence.