This study describes the development of an observational tool, the Write Start! Writing Assessment, created to provide descriptive information on four features of preschoolers' writing: forms, directional patterns, intentionality (ways of assigning meaning to marks), and message content. Observational categories were generated from a review of research and then refined through constant-comparative analysis of the writing of 139 low-income, African American children aged 2:6 to 5:11. Children participated in the study from 1 to 3 years. Fall and spring writing samples were collected as children responded to a standard task asking them to write a photo caption. Cross-sectional analyses across seven age bands show the range and relative frequencies of writing categories. A wide range of normal variation was observed within age groups. Growth curve analyses confirmed that children showed significant change in all four writing features over the preschool years. Data from a subsample of 10 children were analyzed longitudinally over 3 years to triangulate findings. Although children moved toward more conventional writing, inter-individual variability in initial starting points and pacing of transitions into more advanced categories was observed for all four writing features. Intra-individual variability was observed in back and forth movement between less and more advanced writing performances, concurrent use of multiple hypotheses, and differential development of the four writing features. The authors conclude that the descriptive information provided by Write! Start! measure provides a starting point for differentiation of instruction for young writers. (author abstract)