This book constitutes a holistic study of sociolinguistic development among adult second language (L2) learners in a naturalistic setting. Combining results from a cross-sectional and longitudinal micro-development study, it examines the acquisition of both the productive and interpretive skills necessary to engage with the contextually sensitive use of vernacularity in Austria. The studies focus on issues of inter- and intra-individual variation and aim to shed much-needed light on "why" L2 learners acquire sociolinguistic variation, "which" aspects of it they acquire, "which" factors influence it and "when" they acquire it. Drawing on methodological and theoretical frameworks from diverse fields such as developmental and cognitive psychology, psycho- and sociolinguistics, as well as second language acquisition, the book connects topics rarely found in the same empirical piece, including the role of cognitive functioning and socioaffective factors, the temporal specificity of L2 sociolinguistic development in production and perception, and the issue of group-to-individual generalizability. The studies showcased in this work provide ample evidence that late starters of a L2 in early adulthood and midlife can acquire the skills necessary to successfully navigate the complex sociolinguistic variation within the target-language community, but the process is certainly not one without hurdles.