Purpose: This study explored the perceptions of teachers regarding principal support for and understanding of effective writing instruction and whether certain areas of knowledge influence principals' actions and interventions. Research Design: Researchers surveyed teacher participants using the Principal's Support for Writing Instrument and also asked whether their principal was trained in writing as a process. The sample (N = 169) included elementary and secondary rural, urban, and suburban teachers. Findings: An exploratory principle-axis factor analysis with a promax rotation was conducted to determine the underlying structure of the 13 survey items. Results of the factor analysis yielded a two-factor correlated solution. The first factor accounted for 55.79% of the variance and the second for 6.94%. The two-factor correlated structure was confirmed using structural equation modeling. Most fit statistics were observed to be good. Using the subscales as observed variables, a path analysis was conducted using the theoretical latent variables generated in the confirmatory factor analysis to explore the potential causal relationship. The effect of Belief on Intervention was 0.72. Knowledge explained approximately 57% of the variance of Intervention. Conclusions: There is clear indication that principals who have strong knowledge of and belief in effective writing practices act in ways that help teachers do their best work. Those high in knowledge and belief were likewise high in quality actions and interventions. These results hold promise for school leaders and those shaping preservice and professional development opportunities for current and future practitioners. (Contains 2 tables and 2 figures.)