This issue of the Journal of Historical Sociolinguistics aims to contribute to our understanding of language change in real time by presenting a group of articles particularly focused on social and sociocultural factors underlying language diversification and change. By analysing data from a varied set of languages, including Greek, English, and the Finnic and Mongolic language families, and mainly focussing their investigation on the Middle Ages, the authors connect various social and cultural factors with the specific topic of the issue, the rate of linguistic change. The sociolinguistic themes addressed include community and population size, conflict and conquest, migration and mobility, bi- and multilingualism, diglossia and standardization. In this introduction, the field of comparative historical sociolinguistics is considered a cross-disciplinary enterprise with a sociolinguistic agenda at the crossroads of contact linguistics, historical comparative linguistics and linguistic typology.