This article presents the results of a quantitative study in which the complexity levels of dialectal varieties belonging to the South Slavic dialect continuum are measured and analyzed. The sample comprises 919 data points, pertaining to the Bulgarian–Macedonian and Serbo-Croatian dialect continua. Complexity is viewed in this study as a property demonstrating variation across areas and subject to diachronic change which can be associated either with language-internal processes or with language contact. This study discusses which linguistic processes produced varying levels of complexity in the modern South Slavic varieties. In particular, a correlation of complexity with altitude and distance to the Albanian border, two factors which can be associated with degree of isolation versus contact of speech communities, is investigated for a subset of varieties spoken across the areas with bi- and multilingual population. Suggestions on which constellations of societal features could act as determinants of linguistic change are made for several areas within the South Slavic continuum. Particular attention is paid to the contact-related developments in the South Slavic varieties spoken in the areas of intensive past and present contact, such as the west of North Macedonia, the south of Montenegro, Kosovo, and Southeast Serbia.