This is a interdisciplinary study composed of extensive research and detailed analyses of Yunnanese, a Southwestern Mandarin language spoken in Yunnan, China, and Kunming Chinese--one of its major varieties spoken in the city of Kunming. The research work is conducted in three major areas: the language communities, the phonological systems, and the phonological developments in the past six decades. The language communities are discussed from the perspectives of ethnology, sociolinguistics, and dialectology, covering such aspects as history of the civilization of Yunnan and Kunming, the ethnographical and ethno-historical account for the twenty-four ethnic groups inhabiting in Yunnan province, the demographic statistics of these groups, and dialect geography of Yunnanese and its varieties, as well as the members of Southwestern Mandarin subgroup. A language survey has been conducted in some detail on the varieties of Yunnanese represented by one hundred and thirty-five locations with a comparative study of their segmental and suprasegmental structures. A comparative study on the language data representing two different varieties of Kunming Chinese spoken in two different periods of time, i.e., in 1940s and in 1990s, discloses the striking sound changes undergone by this dialect. Analyses of tone sandhi in autosegmental and metrical framework have revealed the edge sensitive characteristic of its tone system, as well as the constrains of tone sandhi imposed by syntactic structure and lexical category.