Online language learning and teaching is a field that has received a significant amount of research attention. What factors could affect student participation in simpler online learning environments has been investigated by researchers, but there has been limited study of factors affecting participation in complex Multi-User Virtual Environments. By using the typical Multi-User Virtual Environment Second Life, three English courses offered by Swedish universities were examined in this thesis. The courses were video-recorded, and selected parts of the recordings were transcribed. The transcribed recordings were complemented by author(s)’ observation, participants’ reflection, an online questionnaire and an online interview. Participation from the courses was measured both quantitatively and qualitatively. Quantitative methods were used to measure, for example, floor space, number of utterances, turn length, number of turns; the qualitative analysis centered on, for instance, utterance functions, discourse analysis, and Conversational Analysis. The results were published in five papers that focused on different central factors affecting participation in Second Life. In this thesis, the findings from those articles are synthesized. Furthermore, on the basis of the findings, a general model of factors affecting participation is presented and discussed to highlight that different factors interrelate and that some factors are particularly important in terms of affecting participation in Multi-User Virtual Environments. These are students’ technical skills, task design, course design, technical support, and Second Life technology. The complex technology also places critical demands on teachers’ technical skills, teaching strategies, and roles that teachers should play. Finally, this thesis argues that it is important to choose a suitable technology for an English course.