Thirty-eight fourth-through sixth-grade teachers (28 female; 10 male) in urban schools in Chile were interviewed to determine how teachers' preparation to teach writing and their knowledge of how to do so acted as facilitators and/or barriers to classroom writing instruction. Teachers overwhelmingly identified their preservice preparation as a barrier to teaching writing, although several teachers viewed this mode of preparation as a facilitator. For teachers who expressed views about in-service and personal preparation as well as teaching experience, they almost unanimously indicated such modes of preparation facilitated classroom writing instruction. Teachers identified five forms of teaching knowledge that acted as facilitators and barriers to writing instruction. This included knowledge of how to differentiate writing instruction, assess writing, support the writing process, teach writing genres, and apply pedagogical teaching methods. A lack of instructional knowledge in each of these areas served as a barrier to classroom writing instruction, whereas knowledge about each acted as a facilitator. In addition, a lack of knowledge about how to teach fundamental writing skills acted as a barrier for some teachers. Implications and directions for future research are provided.