Using a sociocultural framework, this dissertation investigated the writing processes of 31 ESL learners in an EAP context at a large North American university. The qualitative case study involved one of the four major writing assignments in a required first-year composition course for ESL students. Data were collected from four different sources: (a) A semi-structured interview with each participant, (b) process logs kept by participants for the entire duration of the writing assignment, (c) classroom observation notes, and (d) class materials. Findings that emerged through analyses of activity systems, an analytical framework within Vygotskian activity theory, indicate that L2 writers used various context-specific, social, and cultural affordances to accomplish the writing tasks. The study arrived at these findings by creating taxonomies of the six activity system elements - subject, tools, goals, division of labor, community, and rules - as they were realized by L2 writers, and examining the influence that these elements had in the process of composing. The analysis of data helped create categories of each of the six activity system elements. To illustrate with an example, the categories that emerged within the element division of labor were as follows: (a) Instructor, (b) friends and classmates, (c) writing center tutors, (d) family members, and (e) people in the world. The emergent categories for each of the six activity system elements were then examined to determine if their effects on L2 writing were positive or negative. Overall, the findings of the present study validate arguments related to the post-process views that an explanation of L2 writing processes solely based on cognitive perspectives provides but only a partial picture of how second language writing takes place. In order for a more comprehensive understanding of L2 writing one must also account for the various social and cultural factors that play critical roles in the production of L2 texts.