This study considers how English educators can catalyze learning within secondary English education programs to help preservice teachers define and re-examine what counts as writing in the 21st century, and to apply this knowledge in their future school contexts. In it, I explore how preservice teachers develop, articulate, and enact conceptual frameworks of 21st century writing and teaching as they transition from university courses into secondary English classrooms. My research offers critical perspectives to English educators about how they can support preservice teachers in developing reflective mindsets and theoretical knowledge that will enable them to enact digital and multimodal writing within a range of contexts in lasting, evolving, and rhetorically transformative--not just technical and transactional--ways. The dissertation comprises three article-length manuscripts which explore common themes of learning transfer, metacognition, and reflective practice to support preservice teachers in developing and implementing knowledge about 21st century writing principles in context. The research explores preservice teacher learning across a range of sites and through methodologies including an analysis of preservice teachers in an English methods class (Chapter 2), a collective case study of four student teachers (Chapter 3), and a single case study of one participant in her first teaching site (Chapter 4). Central to my approach was situating participants as co-learners and co-researchers, acknowledging the study itself as an intervention worthy of investigation. Taken together, the three chapters point to limited metacognitive interventions as a tool for preservice teachers to name what they know and reflect on their experiences, leading them to arrive at nuanced and adaptable understandings of and confidence in implementing 21st century writing and teaching practices. I offer suggestions to English educators about how to integrate structures that will help preservice teachers become both flexible and deliberate in enacting, advancing, and advocating for relevant and evolving 21st century writing pedagogies. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]