This systematic review of 46 published articles investigates the constructs employed and the meanings assigned to writing in writing-to-learn assignments given to students in science courses. Using components of assignments associated with the greatest learning gains--meaning making, clear expectations, interactive writing processes, and metacognition--this review illuminates the constructs of writing that yield conceptual learning in science. In so doing, this article also provides a framework that can be used to evaluate writing-to-learn assignments in science, and it documents a new era in research on writing to learn in science by showing the increased rigor that has characterized studies in this field during the past decade.