Many different, and even opposite, meanings are ascribed to the term 'sources' of international law. The author of this work goes back to the meaning of the term 'source' in general (spring or well) and analyses in detail the various sources of international law. He first explains the sources of general, and then those of particular international law. He starts with general principles of law, which is followed by common features of customary process of whatsoever kind, and then by general and by particular customary law. Custom will be followed by unilateral acts of States and with opposable situations in international law which are closely linked with this kind of sources of international law. The explanation ends with treaties in regard to which there are the least doctrinal controversies. The explanation cannot be quite homogeneous. There are still deep doctrinal misunderstandings in respect to general principles of law and of unilateral acts of States. The author therefore offers a critical analysis of representative views of other authors and tries to reach solutions to problems presented. He also gives a systematic explanation of recent pronouncements of international courts and tribunals with regard to customary law, and he examines the specific solutions prescribed in the 1969 Vienna Convention on the Law of Treaties.