Collaborative prewriting discussion (CPD) is one of the most popular activities in second language (L2) writing classrooms. To help students discuss effectively, language teachers may consider a number of factors and conditions (e.g., task instructions, types of writing tasks, language use, students' beliefs, students' proficiency, or group selection) when implementing the CPD task in teaching L2 writing. This article aims to review related research and describes how those factors and conditions mediate the benefits of CPD, providing language teachers with pedagogical implications in the design of CPD tasks in teaching L2 writing.