The biographical method is one of the classic method in qualitative sociology and other social sciences. It can be useful not only in sociolinguistics, but also in linguistics. The concepts of biographical method and linguistic biography are used at different levels of linguistic analysis. A research group of scientific workers, PhD and MA students of the Nicolaus Copernicus University has been conducting research on bilingualism of the Old Believers in Poland since 1999. In our works, the biographical method was used not only as a methodological guideline; biographical materials served as the subject of analysis both in sociolinguistic and purely linguistic terms. Moreover, linguistic and sociolinguistic material was collected with a help of biographical interviews (if possible, also personal documents and memoirs). The aim of this article is an attempt not only to organize terminology and systematize methodological approaches, but also to present the advantages of using the biographical method in the process of gathering the material its analysis against the example of research on various aspects of bilingualism of Old Believers living in Poland. Thanks to long-term contact with informants and a longitudinal perspective, it was possible to trace the development of individual idiolects for about 20 years, to carry out diachronic analyzes of changes in phraseological resources, as well as to characterize other phenomena occurring in the conditions of the Polish-Russian bilingualism of the Old Believer dialect island in Poland.