Collaborative writing (CW) is on the rise in primary English as a foreign language (EFL) education, which warrants further research on the opportunities and challenges of CW with young learners. The present thesis aims to contribute with knowledge about pedagogical implications of CW in EFL instruction at primary school. More specifically, affordances of CW are explored in four empirical studies targeting EFL teachers and young learners (aged 11–12) in Swedish school years 4–6. These studies build on qualitative interviews with teachers, audio- and screen recordings of young learners’ computer-mediated CW in pairs, post-task interviews with CW pairs and handwritten narrative learner texts produced individually and collaboratively. The thesis is theoretically informed by affordance theory, while also drawing on teacher beliefs, social deontics and systemic functional linguistics (SFL). Methods employed include reflexive thematic analysis, and transitivity and appraisal frameworks, which are tools situated in SFL. The findings highlight affordances of CW from teachers’ and young learners’ perspectives, operationalised as actions taken that shape EFL learning opportunities. Such affordances encompass (meta)linguistic, social and affective aspects. (Meta)linguistic affordances, such as the potential of CW to enhance vocabulary knowledge, are emphasised by teachers as well as by young learners. The comparative analyses of the narrative texts also reveal linguistic affordances of CW, since texts written collaboratively use evaluative language to a greater extent than individual texts and are therefore potentially more engaging to a reader. When constructing the narrative texts, (meta)linguistic affordances offered by peer/teacher support and Google Translate are frequently enacted by the young learners. The use of this digital tool can, however, encourage or discourage pairs’ joint discussions about language, and their critical awareness of suggestions made by Google Translate varies greatly. In addition, peer support and the CW situation are affected by social relationships formed either prior to or during CW, which fluctuate in interaction and are shown to influence EFL learning opportunities. As an example, the teachers voice concerns about how to pair young learners to stimulate language learning and ensure collaboration, and classroom data confirm this challenge. The teachers further assign CW a motivational role in primary EFL education, which exemplifies a prominent affective factor and positions CW as not merely a (meta)linguistic practice. The results thus indicate that CW is a multifaceted activity and, as such, may require considerable pedagogical efforts on behalf of the teacher.