The system of the Tigris-Euphrates Rivers is one of the great river systems of southwestern Asia. It comprises the Tigris and Euphrates Rivers, which follow roughly parallel courses through the heart of the Middle East. The lower portion of the region that they run through is known as Mesopotamia, was one of the cradles of civilisation. There are several environmental factors that govern the nature of the two rivers and shape the landscape the two rivers running through. Geological events create rivers, climate monitor the water supply, the surrounding land influences the vegetation and the physical and chemical features of water. The Tigris-Euphrates system runs through the territory of four countries, Iraq, Iran, Turkey and Syria. Therefore, any scientific approach to the environment of these two rivers should include the natural history events in these countries. The book "Tigris and Euphrates Rivers: Their Environment from Headwaters to Mouth" will be divided into nine parts. These parts deal with the issues of the environment, the status of the flora and fauna, the abiotic aspects, ecology, hydrological regime of the two rivers, the biotic aspects. Water resources, stress of the environment, conservation issues. Since the book of Julian Rzoska "Euphrates and Tigris Mesopotamian Ecology and Destiny" in 1980, no book or major reference has been published that includes between its cover the facts and information that the present book will present. Therefore, the importance of the present book falls in stating the present status of the environment of the two rivers and the comparison of their environment between now and that of 37 years ago as given by J. Rzoska (1980). The recent studies showed that there are a large number of natural and political events that happened within the last three decades in the area of the Tigris-Euphrates river system that for sure have done a great change to the environmentof the two rivers and consequently changing the biological and non-biological resources of the two rivers. This book will be a reference book to both Academic and students across the Middle East in different disciplines of knowledge to use in their researches on Tigris-Euphrates river system. The scholars interested in this area will use this book as a guide to compare this freshwater system with other areas in Asia and the world.