This thesis examines tax disputes adjudicated by investor-state tribunals. I argue that the nature of taxation – a compulsory levy – is unlike any other state regulatory measure such as an environmental or a public health measure. I suggest that tax-related investment disputes constitute a unique category of foreign investment disputes, and that investment tribunals should recognize them as such. Not all arbitral tribunals, however, acknowledge the distinct character of taxation which leads to inconsistent jurisprudence. Apart from the nature of tax, tax carve outs are another important factor that distinguish tax- related investment disputes from other investment disputes. I show that tax carve outs are included in IIAs in order to prevent overlap with tax treaties. Tax carve outs determine the scope of the tribunal’s jurisdiction, and limit the substantive obligations that are applicable to a state’s taxation measures; but since tax carve outs are worded differently and vary in scope, each arbitral tribunal needs to establish its jurisdiction carefully. While some tribunals have been careful to clearly demarcate their jurisdiction, others have been imprecise. An analysis of the arbitral awards reveals that the approaches of investment tribunals in tax- related investment disputes differ as to expropriation, fair and equitable treatment, and non- discrimination. For expropriation claims, some tribunals regard tax measures as a unique category and correctly set a high threshold for taxation measures to qualify as expropriation. With respect to non-discrimination and fair and equitable treatment claims, investment tribunals tend to assimilate tax measures to all other measures. These differences reflect not only the distinct nature of the obligations, but also tribunals’ failure to recognize that tax-related investment disputes constitute a unique category. In the future, it is likely that the lower threshold for fair and equitable treatment and non- discrimination will lead investors to rely primarily on these standards in tax-related investment disputes. Additionally, international tax law (ITL) has played a limited role in tax-related investment disputes. Investment tribunals, instead, prefer to rely on WTO jurisprudence in their analysis and findings. I argue that there are general principles of law in ITL which tribunals should include in their analysis.