The River Nile, the most famous river of the ancient world, is the dominant geographic feature of northeastern Africa and the longest river on Earth. At the point of discharge of the Nile into the Mediterranean, the great Nile delta has formed and furnishes the most fertile area for cultivation in the Egyptian territory. The delta is embraced by two large branches of the Nile (the Rosetta and Damietta branches and their promontories), as the northward flowing river bifurcates near the city of Cairo. Both the Rosetta and Damietta branches discharge freshwater directly and indirectly into the Mediterranean Sea to form the Nile estuary (also known as the Nile delta coastal area). Fluctuations in both quantity and quality of the Nile water reaching the Mediterranean, especially as a result of the Aswan High Dam (AHD) construction in 1965, have profoundly influenced the morphometry and hydrology of the Nile, and the ecological characteristics of the river and the surrounding marine environment. This chapter intends to highlight the range of characteristics of the Nile estuary and the main factors influencing them since the AHD construction. To this effect, the geography, hydrology, and ecology of this river-delta-estuary- coastal marine system will be described and illustrated, and recent numerical simulations of its hydrodynamics and ecosystem features will be discussed. The concluding remarks forecast future trends in the development of the Nile estuary and its vital role in the ecology of the Mediterranean Sea.