This book will serve as a basis for a developed study of the morphological classes of the irregular verbal and nominal forms and the main phonological changes that occur in the Arabic language. For this purpose, the author presents and studies the forms of the doubled, hamzated, verb with first, 2nd or 3rd weak radical, and the verb that is doubly weak. The existence of a doubled segment, a hamza or a weak segment in the word can result in different phonological changes which lead it from one base form to a derived form. The author adopts an original model which takes into consideration the succession of different segments occurring in one word, and in some cases, in two words following each other. With its numerous references, this comprehensive work is likely to stimulate further research and is an invaluable resource for intermediate and advanced students of Arabic and for anyone interested in the Arabic language and linguistics.