The rules of international humanitarian law of armed conflict are codified in a rather extensive body of treaty law. In addition, extensive research has been conducted into the rules of customary international humanitarian law. The author of this contribution will argue that there is another important source of positive international humanitarian law: principles of international humanitarian law. In this chapter, the role of the principles of international humanitarian law, the functions they perform and their legal significance as a source of international humanitarian law will be assessed. With general public international law as its starting point, the chapter discusses the sources of international humanitarian law. It explains the important role of the Martens Clause and provides examples of how the principles of international humanitarian law may be applied in contemporary armed conflicts.