This study investigates an elective English class, in which students in grades 10-12 collectively read and collaboratively wrote fantasy fiction in four groups. The purpose of the class was to have students consider the choices fantasy and science fictions writers, directors, and video game designers make when creating a fictional world. The students read fiction, watched movies, and discussed video games to consider how storyline continuity is established and maintained across media. Each small group of students created their own fictional world housed on a wiki, which consisted of collaborative writing, created and found images, and digital cartography. This study focuses on how the teacher, the students, and I supported and encouraged social practices related to collaborative writing, how the students worked together and apart with a shared set of tools to coordinate their collaborative writing, and how positional identities of authorship were related to how and why students wrote collaboratively. This study is situated at the intersection of three areas of research: understanding relationships among students' in- and out-of-school literacy practices, understanding how students accomplish collaborative forms of writing with online digital tools, and understanding how students' positional identities are related to authorship. The study draws on three complementary theoretical frames that align with these three areas: New Literacy Studies, mediated discourse theory, and positioning theory. The methodology used is grounded in mediated discourse theory and includes two levels of analysis: at the macro level, nexus analysis is employed to understand what discursive and nondiscursive social practices are constructed and enacted and what relationships among those social practices support or thwart the collaborative writing; at the micro level, mediated discourse analysis is employed to understand how students take up available mediational means to take social action in order to accomplish the collaborative writing and how students position themselves and one another as authors, animators, and principals of the wiki pages that constitute the Building Worlds Project. Findings indicate that the students' histories with writing shaped what social practices they did and did not enact related to the writing of the project. The students demonstrated a concern for the ownership of their own and each other's wiki pages. This concern for ownership was directly related to the most durable social practice of "posting writing to own wiki page" which was commensurate with school-based social practices related to writing that the students reported in interviews. Findings also indicate that students' social interaction, social relationships, and positional identities of authorship shaped how and why they took up mediational means in the ways they did when taking social action related to the writing of the project. This study has implications for the field of literacy studies and writing research by demonstrating how students took up a digital tool, i.e., a wiki, to write collaboratively in ways that are commensurate and incommensurate with new literacies. This study also provides insight into how writing histories shape how writing is accomplished and how students negotiate authorship within social interaction and existing relationships. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]