Abstract : This thesis analyses sound changes that affected the lateral approximant inherited from Latin in Occitan dialects, in the Romance languages, and in a number of other languages from around the world. Chapter 1 gives a comprehensive overview of the research carried out on the lateral approximant; it discusses articulation and acoustics as well as abstract representations of the sound. Chapters 2 to 5 are devoted to specific sound changes which occurred in Occitan dialects at different points in time. These developments are systematically compared to similar phenomena in Romance and other languages. In chapter 2, I discuss the vocalisation of the dark lateral in preconsonantal and word-final position as well as intervocalically. It is argued there that Occitan and more generally Romance followed an unexpected pathway towards vocalisation, which cannot be explained by phonetic factors alone. Chapter 3 deals with palatalisation of the lateral in onsetclusters. Rather than in articulatory assimilation, I propose that the origin of this sound change is to be sought in the frication which accompanied the obstruent + lateral onset clusters. Rhoticisation of the lateral, and its opposite, lambdacisation of the rhotic, is the topic of chapter 4. In this chapter, I discuss duration factors in these sound changes and present experimental evidence to substantiate the idea that duration plays an important role. Finally, chapter 5 looks at thedevelopments of the Latin geminate lateral in Gascon and other Romance dialects; according to common opinion, the Latin geminate lateral underwent a retroflexion process, and I discuss how this might have been possible from a phonetic point of view.