The last several decades have shown a growing number of cases in international human rights treaty bodies, notably the European Court of Human Rights and the UN Human Rights Committee, that assess the compatibility of States' decisions to expel aliens with the right to family and private life. In these cases, the treaty bodies conduct a case-by-case proportionality assessment with a view to strike a fair balance between the interests of the States that order the deportations and the rights of the individuals involved. While these developments signalled a recognition to the right to family and private life as a possible substantive limitation to the expulsion of non-nationals, the scope and application the limitation remained blurry and controversial. This is manifested in the ILC Draft Articles on the Expulsion of Aliens adopted in 2014. Draft Article 18, which deals with the limitation, singled out the family life aspect (omitting private life) and failed to explicitly mention the need to conduct proportionality assessment as a minimum requirement. Against this background, this thesis examines the legal basis, the extent of recognition, application and challenges of the substantive limitation; by exploring the nature and interaction of the relevant international legal norms, through a comparative analysis of European and USA's approach on the matter and a review of the practice of selected treaty monitoring bodies. The analysis demonstrates that proportionality assessment as a minimum requirement in expulsion cases that affect family and private life is derived from the limitation regime of the relevant human rights provisions and is further implied in the traditional safeguards against arbitrary expulsion. However, as the power to expel aliens continues to be perceived as one of the last bastions of State sovereignty, the human rights-based proportionality analysis faces challenges on multiple levels.