Is the application of international law unjust in prominent case law of the Court of Justice of the European Union? In answering this question the thesis explains that Justice comprises both legal certainty, which seeks clear rules to govern future conduct, and [j]ustice, which seeks the correct outcome in an individual case. There is a frequent tension between these aims. Moreover, there is not an accepted model of Justice which resolves how to balance legal certainty and justice. Any loss in one aspect may be justified by a gain in the other. On this basis the analysis of legal certainty and justice is divided in the thesis. Legal certainty can be measured objectively, but the appropriate level of legal certainty which any legal system should achieve is contested. It is argued that legal certainty in internal case law - case law not applying international law - is the appropriate measure of legal certainty in external case law - case law applying international treaty and customary international law. This is implicit in scholarship and has been expressly approved by the Court itself. The thesis will identify that external case law is les certain than internal case law. [j]ustice cannot be measured objectively and is contested in its substantive or 'thick' form. However, formal, or 'thin' justice, which is encapsulated in the obligation to treat like cases alike and unalike cases unalike to the extent of their unalikeness, is widely accepted. Accordingly, the thesis compares internal case law, which reveals strong and consistent conceptions of justice across EU law, to external case law. It identifies differing justice conceptions in external as copared to internal case law. Based on a 'thin' model of justice these justice modifications must be accounted for based on the context of applying international law; the extent of the unalikeness. It will be identified that unalikeness in the context of applying international law cannot account for the justice modifications in external case law and a fortiori the lower levels of legal certainty. Accordingly, this thesis endeavours to show that the application of international law in prominent case law of the Court of Justice of the European Union in unjust.