This study examined preschool teachers' writing knowledge and how this knowledge relates to classroom writing practices. Head Start teachers (N = 47) across two states participated by completing a knowledge questionnaire in which they responded to three vignettes and samples of preschool children's writing. Teachers' writing practices were gathered and coded from half-day video observations. Questionnaire responses were iteratively coded, first, using a set of a priori developed codes, derived from well-established theories of writing and including subcomponents of writing: handwriting, spelling, composing, and print concepts. Then responses were open-coded using an iterative process. Responses to the vignettes revealed that teachers' knowledge of early writing development generally aligns with research-based conceptualizations for handwriting and print concepts, but less so for spelling and composing. Teachers varied widely in the components they discussed, with clear patterns across the three writing samples. Observations of teachers' practices revealed that teachers primarily enacted practices focused on children's handwriting skills and engaged in scaffolding strategies designed to make writing easier for children more frequently than modeling or expansion strategies. Most instructional strategies were considered low quality as teachers were observed doing much of the cognitive or physical work of writing. Teachers' knowledge and practices were related. Teachers demonstrating higher knowledge complexity (i.e., discussed more writing components in their responses) engaged children in more writing and offered higher quality supports reflecting a wider range of writing components.