The Caspian Sea, the biggest lake of the world, is of great importance to its neighboring countries, including Iran, Azerbaijan, Russia, Kazakhstan and Turkmenistan.Therefore, planning for and the management of its natural resources are what these countries should carefully deal with. In this article, the author gives an account of some geographical features of the Caspian Sea and the rivers which support its water supply. Moreover, its major problems, ie its pollution and the uncontrolled exploitation of its resources by humans, and its water fluctuation that sometimes damages the surrounding lands are mentioned. Besides, the Qarah Boghaz Gulf, located on the eastern part of the sea, is described and the changes in its amount of water are studied. In conclusion, the author considers the causes of the sea’s transgression and the damages it has to the coastal areas of Iran.