In this study, a substantial introduction has been designed to familiarize the reader with the main characteristics of the philological discipline. Substantive sections have been devoted to Vulgar Latin as the parent tongue of the Romance languages, to the history of the discipline, and to the historical comparative method. This is followed by a sustained study of the sound changes that eventually lead from the relative unity of Vulgar Latin to the diversity of the Romance languages. Sound laws are clearly formulated and each rule is illustrated by examples. Factors that interfere with regular developments are discussed: analogies; learned Latinizing influence; borrowings; etc. The presentation of phonology is coupled with an important lexical dimension, which serves primarily to illustrate the fragmentation process that has shaped the vocabulary of each individual Romance language.