Very few studies focus on how English as a second language (ESL) students’ agency and their unique histories as an integral part of the social cultural environment influence his or her participation in computer-mediated peer response tasks, particularly in a multimedia-based synchronous communication environment. Considering each ESL student as an active agency with unique historical bearings, the dissertation investigated ESL students’ participation in computer-mediated peer response (CMPR) tasks that used instant messenger (IM) as the communication technology between students from the cultural historical activity theoretical (CHAT) perspective, which views all human interaction as a dynamic developmental process. A case study approach was adopted to collect qualitative data from five ESL students enrolled in a level-4 academic writing class in summer, 2006. The entire study spanned from May to August. Each of the five participants participated in three CMPR tasks throughout the semester. Data were collected from multiple sources including a demographic survey, IM chat transcripts, the researcher’s participative observations, participants’ on-screen and off-screen behaviors, their first and second writing drafts, interviews, the researcher’s reflective journals as well as documents collected in each instructional modules. Both within-case and cross-case analysis were used to identify emergent themes. Specific methods included constant comparison method, content analysis, revision analysis, and CHAT analysis. The findings showed that ESL students had multiple and heterogeneous motives and goals within and across CMPR tasks. Some motives were learning-oriented while others were nonlearning-oriented or even entertainment-oriented. The use of IM not only triggered each student’s motive and goal formation and shift, but also transformed his or her particular behaviors and the relationship established during each CMPR session. ESL students’ online contributions were strongly influenced by the pair’s IM communication styles and competences rather than the task types or their motives. Students also developed new perceptions about CMPR tasks, which shaped and were dialectically shaped by their participatory behaviors in each task. Conflicts and tensions existed within and between both contemporary and historical activity systems in which each student was involved. Those who actively sought solutions to the conflicts developed new knowledge and skills such as writing an exploratory essay and competences of conducting CMPR tasks. Those who ignored the conflicts experienced scarce expected development.