This paper aims to examine theoretical foundations of and empirical evidence for collaborative ESL writing pedagogy and suggest it as an alternative model to traditional approaches in writing instruction. The history of research on collaborative tasks in teaching writing is relatively short and it received explicit attention in the mid 1980s (Doheny-Farina, 1986; Odell, 1985). The need for research in collaborative writing in ESL/EFL classroom is justifiable from philosophical, psycholinguistic, pedagogic, and psychological points of view. A brief history, types, and essential elements of collaborative ESL writing instruction are presented. In an attempt to investigate how these rationales and elements are realized in actual pedagogic processes, a case study was conducted based on an observation of a collaborative task-based ESL writing class at a US university. Classes for a full thematic unit, consisting of eight hours, were observed. A variety of carefully designed collaborative activities implemented in the ESL writing class illustrated clear benefits of collaborative writing pedagogy. Pedagogical implications for writing classes in EFL contexts are also discussed.