Learning an additional foreign language (usually referred to as a third language, L3) after English (L2) in formal settings seems to remain relatively unsuccessful in the European context (European Commission 2012), despite the reported advantages from extensive language learning experiences. Against this background the present thesis explores the potential benefits of a teaching approach focused on writing strategies and metacognitive reflections for L3 writing. The study investigates the influence of an intervention concerning the writing of argumentative texts in L3 German. Two classes with 15- and 16-year-olds from a Swedish upper secondary school participated. One class received writing instruction, the other one no special treatment. Texts were collected at three points in time. Additionally, seven students from the intervention class, the “focus students”, attended five individual writing sessions, in which their argumentative writing was recorded with keystroke logging and screen-recording software. After each writing session, individual stimulated-recall interviews were conducted. The development of L3 writing throughout the course of the intervention was investigated from three different perspectives: writing products, writing processes and metacognitive knowledge. Text quality was studied on the basis of a selection of class texts, which were scored both analytically and holistically. The writing processes were examined by automated and manual analysis of the conducted log- and screen-recording files. Metacognitive knowledge was analysed based on transcripts of stimulated-recall interviews. The findings show that only the texts of those students, who attended both the intervention and the individual writing sessions, including reflective interviews, improved in quality during the intervention. Further, these students slightly increased their writing fluency through higher text production rate and a shift from intense online-source use to more revisions. The investigation of metacognitive knowledge revealed a number of learner- and language-related variables, which seem to be decisive in understanding the interplay between learners’ attitudes, performance and strategic behaviour. The results suggest that writing practice is crucial, but learning to write can be further enhanced by including writing strategy instruction and metacognitive reflections in the foreign language classroom.