Process writing research began with adult writers, eventually expanding to include school age children and more recently, emergent writers. Research at the early childhood level has often been directed at specific aspects of writing development rather than an examination of process writing development. This study used pre-existing writing samples to examine writing development in kindergarten and first grade over the course of the school year following the district-wide implementation of a writing process based intervention. The intervention utilized a writing workshop approach to teach the writing process with the addition of two elements: picture plans were used to support emergent writers’ prewriting plans, and teachers focused on a single teaching point to target writing instruction. Beginning and end of year samples from 138 kindergarten and 106 first-grade students from three elementary schools in a medium-sized, public school district in the southwestern United States were used for this study, yielding a total of 488 samples. The samples were scored to investigate the change over time on four outcome measures: quantity of words produced, attributes of prewriting picture plan, evelopmental level, and handwriting. In addition, the impact of fidelity to the intervention features was explored in relation to the four outcome measures. Fidelity to implementation was scored on each of the 10 separate aspects of the intervention: student choice for topics, reading-writing connections, prewriting, peer conferencing, teacher conferences, minilessons, revision, editing, publishing, and modeling. Overall, the study found that the greatest change over time in kindergarten and first grade was in the developmental level. There were also large effects for quantity of words produced and handwriting. A regression analysis was conducted to determine which aspects of the intervention feature were most critical to early writing development. Student choice had a significant positive association with all four dependent measures. Minilessons had a significant association with developmental level and handwriting; other significant positive associations included revision with quantity of words produced, and editing with planning. The findings suggest these features of writing workshops should be included in interventions designed to foster early writing development.