This paper focuses on how teachers mediate wiki collaborative writing activities, and the impact of their mediations on students' collaboration. It is based on a study conducted with three English as a Foreign Language (EFL) teachers and their students (aged 17-18 years) at two government-funded girls' high schools in Kuwait. The selected groups of students, supported by their class teachers, carried out wiki collaborative writing activities over a period of eight weeks. The writing activities were tracked from conception to completion, by closely analysing the wiki discussion and edits, in order to explore the process of interaction. This was triangulated with interview data with teachers and students to gain deeper insights. The results show that, despite the fact that all groups of learners and teachers received a similar type of training and worked on the same activity, there were some variations in the level of their collaboration. Teachers played an essential role in shaping the way that learners collaborated; i.e. some teachers' behaviours promoted collaboration, while others hindered it. Although it can be acknowledged that the wiki is a powerful tool for student-centred collaborative writing, it can also be argued that the role of the teacher is indispensable, and the right kind of teacher intervention is critical to the success of collaborative writing, especially in educational contexts that are similar to Kuwaiti schools. Therefore, professional training is required in order to raise teachers' awareness of effective pedagogy in supporting wiki-mediated collaborative writing activities. [For the complete volume of short papers, see ED572005.]