Over the past 40 years, second language educators and assessors have come to the realization that investigating the process of writing can shed light on language teaching, learning and assessment practices (Odendahl & Deane, 2018). What L2 writers do and think while writing can provide links between the task, the related construct and the cognitive mechanisms the learners engage in. While from a pedagogical perspective, such information can help language teachers provide learners with more accurate feedback, in assessment contexts, writing process information can help language testers to ensure that performance-based score inferences are truly linked to not only the linguistic knowledge of examinees, but also to the functions of mental architecture intended in a specific task (Purpura, 2013). Despite the abundance of writing process research in L1 and L2, most studies suffice to a mere list of strategies and the observed phenomena (Manchón et al., 2007). In light of the demand for a theoretically-based approach to L2 writing process, this paper first provides a review of L2 writing process research by presenting the most influential theoretical approaches to the conceptualization and investigation of writing process in L1 and L2. Then it evaluates the contributions made to the concept of writing process by reviewing some representative empirical research in the field and delves more deeply into the investigation of writing process in assessment contexts. Finally, it discusses how a stronger, theory-based understanding of L2 writing process can be achieved by means of using more innovative assessment tools such as scenario-based assessment (SBA) in conjunction with computer technology.