After an overview of several directions of research on cognitive and social processes in writing, this article presents a model of “co-regulation” of writing activities in the classroom. Co-regulation is defined in a situated perspective as the joint influence on student writing of sources of contextual regulation (structure of the teaching/learning situation, teacher interventions and interactions with students, peer interactions, tools and artifacts) and of processes of self-regulation. This conception is illustrated by the results of research on a writing activity in 5th and 6th grade classrooms. The research concerns two aspects of the co-regulation of students’ writing: (1) the role of whole-class discussions in the emergence of taken-as-shared meaning regarding the writing task and the influence of these discussions on the revisions subsequently carried out by students; (2) the articulations between self-regulation (reflected in revisions students carry out individually on their own drafts) and regulations resulting from peer interaction (reflected in revisions made during dyadic interaction). The conclusions drawn from this research are discussed with respect to their implications for writing instruction.