The purpose of this paper is to describe and analyze the challenges and opportunities for the development of international environmental law. This article is a brief description of "international environmental law" as a new branch of public international law. Further, it attempts to provide some knowledge on the sources and principles of international environmental law. The environmental problems in the contemporary world are amongst the most urgent that requires an immediate collective response from the states. Since the environment is a transboundary issue, a single country's effort is not enough, but all countries have to collaborate. In this regard, the solution may be twofold: either work together on having an international instrument or at least give similar concern to environmental problems in domestic environmental law. As we all are living together on this planet, the best way is, yet, working together on implementing international environmental law.