Internally Displaced Persons ('IDPs') outnumber refugees by two to one and often have the same fears, needs and wants as refugees recognised as such under international law. However, refugee status entails international protection, while IDPs are left to the protection of their own state, which may, but by no means necessarily, be the very entity that has forced them to flee in the first place. In recent years, there have been significant developments in the realm of IDP protection. This includes the conclusion of two regional treaties on the protection of IDPs, the development of relevant soft law instruments, and the reformed 'Cluster Approach' of humanitarian response. Although the increased focus on IDP protection is a welcome development, the UNHCR has expressed the fear that 'activities for the internally displaced may be (mis)interpreted as obviating the need for international protection and asylum.' This thesis represents the first legal analysis of the relationship between the protection of IDPs and International Refugee Law. It will discuss five key challenges in this respect. First, the challenge of drawing the attention of the international community to the plight of IDPs; second, the challenge of developing an appropriate framework for the protection of IDPs; third, the challenge of ensuring that internal protection is not interpreted as a substitute for asylum; fourth; the challenge of determining the relationship between complementary protection and internal displacement; and fifth, the challenge of ensuring that IDP protection in an inter-agency context does not trigger the application of Article 1D of the Refugee Convention, rendering the Convention inapplicable to the recipients of that protection. This thesis will conclude by setting out the future challenges in the relationship between IDP protection and International Refugee Law, by identifying questions left open for further research, and by illustrating the overall impact and importance of this thesis' findings.