Phonology is the study of the changes that can affect a word, and sometimes even two words, for different reasons relating to a certain implied heaviness, like the existence of two identical letters or two letters that are close to each other or a hamza or a weak letter. The phonological changes are recognized as the assimilation of a letter to another, the substitution of a letter for another, the addition or elision of a letter or vowel and the transfer of a letter or vowel to another position. This study explores in detail when these changes are necessary, allowed or forbidden. The presentation of the rich data, the theoretical analyzes of the phonological elements and the references to the different works from the classical period of the 8th century until our days, offer a detailed and accessible study for both the students and researchers of Arabic.