This work is a comprehensive study of two well-known phonological changes in Arabic: the assimilation and the substitution. The assimilation involves the incorporation of a certain segment into an adjacent one in such a manner that they both form a doubled segment. The segments can either be identical or different. This phenomenon occurs usually in the doubled verbs and their forms and in some cases of perfects of Form V, VI and VIII. It also occurs in some other more unusual cases as in two words following each other. The substitution involves the replacement of a segment for another different one. It occurs if there is in the word a combination of two segments which is deemed as heavy, or if both these segments' points of articulation are close to each other or if they are akin in character. Other more unusual reasons relate to the peculiarity of a dialectal variant, to a verse's metrical exigency or to the exigency of the pause. This book explores in detail many various cases in which these changes are possible, necessary or forbidden. The phonological elements, the theoretical discussions and the coverage of the different works from the 8th century until our days, offer a thorough and accessible study for both the students and researchers of Arabic.