Diglossia is a language situation in which there are two distinct varieties of a language used side by side. One variety, referred to as the High variety (H), is used only on formal and public occasions, while the other one, referred to as the Low variety (L), is used under normal, everyday circumstances. The distance between the H and L are sometimes to the extent that the two varieties are mutually unintelligible. One good example is the diglossic situation held between the Modern Standard Arabic (MSA) and the Egyptian Colloquial Arabic (EA). This article aims to compare the two varieties, at different linguistic levels, to illustrate how these differences have made the two verities mutually unintelligible.