Learning to write in a third language is demanding and according to the European Commission (2012) often unsuccessful in the European context. This thesis shows that a teaching approach incorporating writing strategies and metacognitive reflections helps upper secondary students to master this challenge. The foreign language teacher is encouraged to include the teaching of writing strategies and to invite students to talk about language, their writing and themselves as learners. These recommentations are based on a writing intervention study in a Swedish upper secondary school. The intervention supported the development of skills required for argumentative writing in German as a third language. The students' writing was investigated from three prespectives: writing products, writing processes and metacognitive knowledge. The investigation of metacognitive knowledge revealed a number of learner- and language-related variables, which are important for the interplay of learners' attitudes, performance and strategic behaviour in the language learning classroom.