This thesis deals with the phenomenon of assimilation, a natural phonological process, in classical Arabic. It consists of three chapters: The first chapter of the thesis deals with the Arabic sounds in isolation: their points of articulation and manners of articulation with reference to the views of both Arab linguists and scholars of Tajw{=i}d. Secondary sounds, both accepted and rejected, are mentioned too. A brief discussion is devoted to the distinctive features, with particular reference to those that seem to have been described inaccurately by some early or modern linguists. The second chapter deals with the definition and different types of assimilation. The term idgh{= a}m, which has been rendered as `lq assimilation is, in fact, narrower in application than the English term; other topics and sub-topics in Arabic grammar that subsume aspects of assimilation are also discussed. The direction of assimilation, whether the influence comes regressively or progressively or in both directions is dealt with, with detailed examples, mostly from the Qur'{= a}n. A brief discussion of both complete and partial assimilation is given followed by a discussion of distant assimilation, with particular attention being given to the so called `lq al-Idgh{= a}m al-Kab{=i}r. The last sub-chapter here deals with the history of sound changes in Arabic. The third chapter is the main part of the thesis. It deals with Arabic sounds in combination. Consonants are discussed first, from two points of view: the occurrence or non-occurrence of certain consonants with each other within the same roots, and the influence of certain sounds upon others when these occur adjacently. Vowels are then discussed in detail, particularly with respect to the best-known phenomena associated with them: im{= a}lah, vowel harmony, lengthening and shortening, and tafkhim. Under im{= a}lah, we consider whether the alif al-im{= a}lah is an independent phoneme, a dialectal variant, or an allophone of the actual /= a/. When this alif is likely to occur and when it is likely to be inhibited is also seen. In the section on vowel harmony the question as to whether there is systematic harmony is confronted. In the section on lengthening and shortening, the circumstances in which these two phenomena occur are discussed, with particular attention to the over-lengthened vowels. Finally, alif at-tafkh{=i}m is investigated, to see whether this vowel /bar >/ is an independent phoneme, a dialectal variant, or an allophone of the actual /=a/.