The data allowed us to explore a range of questions about the development and nature of writing proficiency. Specifically, this paper provides a description of the trajectory of development in writing performance from 4th grade to 12th grade. The validity of a single developmental writing scale is examined through a factor analysis (exploratory and confirmatory) of the internal structure of writing performance and changes in this structure from 4th grade to 12th grade. The paper also explores important factors affecting performance, including prompt difficulty, writing mode, and student background (gender, ethnicity, and English language background).