Research suggests that teachers' and students' engagement in formative assessment contributes to students’ writing development by influencing both what and how students learn. However, while research exists on how teachers and students engage in formative assessment in traditional classroom settings, little work has examined the nature of formative assessment when students write together online. This three-study dissertation aims to fill this gap in the literature through a longitudinal examination of two middle school English language arts (ELA) classrooms. This dissertation explored teachers’ and students’ formative assessment process when the same groups of three students engaged in online collaborative writing to compose source-based argumentative essays. In the Study 1, I employ a multiple-case study approach to investigate (1) the common and unique contextual factors that shape ELA teachers’ formative assessment beliefs and practice, and (2) the challenges they face when engaging in ongoing assessment while students write together online in their classrooms. Primary data sources include pre- and post-study semi-structured teacher interviews and post-instruction debriefs. Secondary data sources include field notes from 100 hours of classroom observations. Findings suggest that district-level and classroom-level factors shape teachers’ perceptions of formative assessment and how they adjust their approach over time. In the Study 2, I take a multiple-case study approach to investigate (1) two teachers’ use of formative assessment strategies and (2) two student focal groups’ use of formative assessment strategies during one week-long collaborative writing activity. Primary data sources include field notes from one week of classroom observations in each school context and screencast data that capture group talk and synchronous online collaborative writing in real time. Secondary data sources include portions of pre- and post-study semi-structured teacher interviews. I find that teachers with different approaches to writing instruction focus on different aspects of online collaborative writing during their formative assessment process. What teachers prioritize in their formative assessments, in turn, shape student groups’ own assessments of writing. Student writing groups take up cues from their teachers about what to prioritize when working together, using tools to support source-based argumentative writing or to support their in-group collaboration. In Study 3, I employ a multiple-case study approach to examine (1) four student writing groups’ strategies for formative assessment, (2) the changes they make to their use of strategies for formative assessment over time, and (2) the strategies that support writing improvements to their source-based argumentative writing. Data sources include 33 hours of screencast data and genre-specific rubric scores for each groups’ essay written at two time points. Findings suggest that groups’ talk focused on setting goals, using classroom tools, evaluating their writing and group dynamics, and making informed changes to their texts through revision. Also, groups that prioritized writing effectiveness, rather than writing efficiency, spent more time on revising, using their own assessments of writing for those revisions, and demonstrated improvements in their source-based argument writing. Together, these studies provide a nuanced understanding of the factors that shape formative assessment during collaborative writing activities in middle school classrooms.