Many L2 learners believe they learn their target language by using it extramurally, i.e. outside the classroom. This topic, language learning as a result of engagement in L2 use, has garnered increased attention in the last decade. However, there is still a distinct lack of research. To date, most research offers snapshot studies of single groups of learners at similar L2 proficiency levels, and English is by far the most commonly researched target language. The present study quantitatively investigates the relationship between extramural Japanese and general Japanese language proficiency among adult learners studying at university in Sweden. It aims to do so both cross sectionally, using five different Levels of proficiency to investigate the moderating impact of proficiency at a macro level, and longitudinally, in order to move beyond correlations and provide empirical data on causal direction. In total, data were gathered from 168 individuals, many of whom participated in the study at different Levels, during a period of two and a half years. Data were analysed using non-parametric correlations and Bayesian regression. Results show that the participants engaged in extramural Japanese for considerable amounts of time every week. Some forms of Japanese language use (e.g. reading text and speaking face-to-face) related to general Japanese language proficiency, whereas others did not (e.g. listening to music and watching video with TL subtitles). These relationships were largely positive, although some relationships were negative or mixed. Proficiency level was a moderating factor, and different forms of language use had varying relationships with general Japanese language proficiency depending on participants’ Levels of study. In addition, the results suggest that extramural L2 use influenced proficiency but proficiency did not influence extramural L2 use. This is among the first studies to show such an effect empirically. Overall, the study makes significant contributions to our understanding of the connection between extramural L2 use and proficiency, which has implications on the applicability of results to other languages. Furthermore, the study also provides an innovative research framework for investigating extramural L2 use, which should be robust and valid for any target language. This is a doctoral thesis in Asian Languages and Cultures at Stockholm University, Sweden 2023