Wiki-based collaborative writing has been increasingly implemented in second language classes. This chapter illustrates two small group writing projects conducted via Wikispaces (http://www.wikispaces.com/) and explicates how the distinctive wiki functions (i.e., “Discussion,” “Comments,” “Edit,” and “History”) mediate collaborative writing processes in the EFL and ESL contexts. The results reveal that wiki “Discussion” and “Comments” enabled the students to engage in the negotiation of writing tasks, as reflected in content discussion, task management, technical communication, language negotiation, and social talk. Transparency of wiki “History” records motivates students to be actively involved with recursive writing processes through multiple rounds of writing and revising using “Edit,” namely addition, deletion, reordering, rephrasing, and correction. The affordances of other wiki modules such as “Projects” and “Writing resources” are reported. The results within the EFL and ESL settings are also briefly compared to illustrate how the wiki tool is used by particular groups of English language learners. This chapter supports wiki small group writing as an innovative way to engage students in collaboration and learning during collaborative writing tasks in both EFL and ESL learning contexts.