The need for supporting student writing has received much attention in writing research. One specific type of support is feedback--including peer feedback--on the writing process. Despite the wealth of literature on both feedback and academic writing, there is little empirical evidence on what type of feedback best promotes writing in online environments. This article reports on research that tries to determine what type of feedback best improves the quality of collaborative writing and what the effects of feedback are on student learning in an environment based on asynchronous written communication. The results reveal that concerning the type of feedback, epistemic feedback or epistemic and suggestive feedback best improve the quality of collaborative writing performance. The nature of the feedback-giver (whether teacher feedback or teacher and peer) makes a difference to the final text only when the feedback is epistemic, or epistemic and suggestive.