There is a growing scholarly interest in the issue of unconstitutional constitutional amendments. Generally speaking, this issue concerns whether there should be some limits to constitutional amendments and whether courts should control those limits. In this sense, unconstitutional constitutional amendment exacerbates the debate concerning the legitimacy of judicial review qua institution, and moves the discussion one step further. The rise in interest among scholars of the issue of unconstitutional constitutional amendments derives from the fact that constitutional amendments are sometimes used as an instrument by authoritarian governments to achieve their aims. The judiciary in various jurisdictions gives negative or affirmative responses to this instrumentalization of constitutional amendments by reviewing the contents of amendments. Thus, judicial review of constitutional amendments on substantive grounds has become a new legal phenomenon, which deserves close consideration. The purpose of this thesis is to contribute to this literature. How is it possible for a court to declare an amendment unconstitutional? Under what conditions can the legality of an amendment be questioned? What substantive considerations outweigh the formal value of a duly adopted constitutional amendment, which is normally regarded as the highest legal source in modern legal systems? What kind of legal theory can explain this practice? These are some of the guiding questions, the analysis of which constitutes the main goal of our work. The analysis is based on the distinction between the aspects of legality and of legitimacy. The legality of a constitutional amendment concerns two considerations. The first is whether the amendment is legally valid in terms of the constitutional norms. The constitutional norms here refer mainly to the procedural requirements or amendment mechanism, which the constitutional amendments have to meet. The second consideration is whether the amendment must conform to some (superior) principles, values etc. Depending on how one conceives of those superior principles, one may approach the issue at hand from the natural law perspective or legal positivism. In the present work, we stick to the legal positivism in accounting for the legality of unconstitutional constitutional amendments. The legitimacy of a constitutional amendment concerns the merit of the amendment according to political morality, namely, whether it is a good or a bad thing, with regard to the value that the constitutional amendment should pursue. Equally, the legitimacy of the substantive ii judicial review of constitutional amendments concerns whether it is a bad or good thing to confer on a court of an extra-ordinary power in a system, which is subscribed to constitutional democracy. This is a normative account of legitimacy, but it is not the only one. Legitimacy may also be approached sociologically, i.e. descriptively. In the latter account, legitimacy is examined on the basis of the political morality, which a legal and political order actually aims to achieve and pursue. These actual aims might be ideal or not (from an outsider and/or insider point of view). We will follow this sociological account in our analysis of the legitimacy of the judicial review of constitutional amendments. The analysis of the issue is carried out through a comparative law perspective. In this respect, three jurisdictions are examined: Germany, India, and Turkey, which provide the most prominent examples of case law concerning the judicial review of constitutional amendments on substantive grounds.