Young children's writing development (i.e., writing occurring in preschool and kindergarten prior to the skilled, fluent writing associated with formal schooling) is an important predictor of later literacy achievement. Current policy movements, such as the Common Core State Standards, invoke increased composing demands, yet research has often focused on children's growing ability to transcribe (e.g., form letters, spell words) rather than their ability to compose text and generate ideas. The current study examined literature from the past 30 years to determine the prevailing operational definitions of early composing and the ways in which composing has been measured to date. Given inclusion criteria, the authors collected and coded articles to determine trends and prevailing approaches (n = 445). Findings revealed that various theoretical and conceptual frames influenced approaches to assessing children's composing, yielding little shared understanding of the nature and development of early composing. Findings also revealed that tasks used to measure children's early composing development have not always aligned with theoretical notions of early writing, nor have they been properly operationalized in psychometrically sound, valid, and reliable ways. Further, analyses of trends across time yielded insight into the evolving conceptualizations and prioritization of particular aspects of early composing, such as the focus on children's use of conventional encoding in a composing task context. Implications for practice are discussed in light of these findings.