​This chapter introduces the environmental gradients that characterize the broader Red Sea habitat. The Red Sea is formed by an actively spreading rift and notably has only one natural connection to the Indian Ocean – a narrow, shallow opening known as the Strait of Bab al Mandab. The resultant isolation undoubtedly plays a key role in shaping the environmental gradients, species endemism, and distinct evolutionary trajectory observed within the Red Sea. While this young ocean is known to be among the saltiest and warmest seas on Earth, there are important spatial and temporal gradients that likely influence the biological communities residing in its waters.