This book is about two important processes in phonology: lenition which is a process whereby consonants become weaker, in the sense of becoming voiced and/or undergoing a diminution in stricture and fortition which is a process whereby a segment becomes 'stronger', or more consonantal, moving up the sonority hierarchy. These two processes are investigated in Persian language system to find the contexts in which they apply according to the framework of generative phonology. I hope the book will be useful in helping the students of linguistics to expand their skills of phonological analysis.