Written in 1878, while the author was a twenty-year-old student in Berlin, Saussure’s only full-length work proposed the existence of two additional sonant coefficients in the Indo-European parent language. Applying the methods of comparison and internal reconstruction to Proto-Indo-European, Saussure argued that the long vowels had developed from a short vowel plus a sonant coefficient. A hypothesis far ahead of its time, his proposal was not confirmed until 1927 when a consonantal phoneme etymologically derived from Saussure’s A was discovered in newly deciphered Hittite, the oldest attested Indo-European language. Not only is the Mémoire a dramatic demonstration of the method of internal reconstruction, but it also paved the way for further developments in historical phonology including laryngeal theory, and may have stimulated Saussure’s later development of structuralism. This reissue includes, as an appendix, Antoine Meillet’s 1913 obituary of Saussure.