► Cognitive and motivation factors can explain important variations in L2 writing processes. ► Goals, self-efficacy beliefs and self-regulatory capacities are influential in the psycholinguistic mechanisms of L2 writing. ► Aptitude, working memory and motivation affect how L2 learners exploit the learning potential of writing tasks. Although the role of individual differences in second language (L2) speech has been extensively studied, the impact of individual differences on the process of second language writing and the written product has been a neglected area of research. In this paper, I review the most important individual difference factors that might explain variations in L2 writing processes and discuss the influence of these factors on how L2 learners exploit the language learning potential of writing tasks. First, the role of cognitive factors will be explored, and recent research investigating the relationship between writing performance and aptitude and working, and phonological short memory, will be presented. Next, the potential role of motivational factors, such as language learning goals, self-efficacy beliefs, and self-regulatory capacities that influence the psycholinguistic mechanisms of L2 writing and the way students learn about the target language through writing, will be explored. The article concludes with a summary of the role of individual differences in L2 writing processes and in learning through writing, and with an outline of a research agenda for future studies.