This article discusses three sets of challenges involved in the assessment of writing from a developmental perspective. These challenges include defining a workable theory of development, developing a suitable construct, and overcoming limitations in technocentric approaches to writing assessment. In North America in recent years, a burgeoning number of scholars have begun to explore the development of writing ability through the lens of knowledge transfer research. This paper discusses limitations in current and traditional conceptions of transfer, proposing a bioecological model of transfer to resolve these issues. Related to issues of transfer are challenges in defining the construct under investigation in the assessment of writing development. Beaufort's (2007) model of expertise in writing is discussed as a promising framework for defining this construct. Related to these challenges are limitations in current assessment technologies. Drawing on Huot's (2002) concept of assessment-as-research, this article discusses possibilities for broadening the range of assessment practices available to assessors of writing. (Contains 1 table.)