This article reports on a comprehensive synthesis of the literature on the role of working memory in second language (L2) writing. It starts with an overview and clarification of the construct and measurement of working memory, followed by an elaboration of major theoretical models informing the synthesized research. The article then presents a synthesis of the methods and results of the 16 studies that have been conducted on the associations between working memory and L2 writing. The methodological synthesis encompasses research design, methods of working memory, measurement of writing performance, methods of data elicitation for writing processes, and data analysis and reporting. The results of the synthesized studies demonstrate that (1) working memory is largely unrelated to overall writing proficiency; (2) it is predictive of specific aspects of L2 composition such as complexity, accuracy, and fluency; (3) the role of working memory varies as a function of genre, proficiency, target structure, instruction type, and task demands; and (4) verbal working memory, phonological short-term memory, visual-spatial working memory, and executive functions (inhibiting, shifting, and updating) have differential associations with the process and product aspects of L2 writing. The methods and results are discussed by identifying trends, accounting for disparities, clarifying confusion, recommending solutions, and proposing new directions.