Writing competence holds an important place in the language development of multilingual writers, and it is regarded as the gatekeeper for ELLs' academic success and advancement to higher-level courses. It is essential that all teachers, not just the English as a Second Language (ESL) teachers, recognize themselves as teachers of linguistically and culturally diverse learners and be equipped with the expertise for teaching writing to foster ELLs' academic language development and second language proficiency. Grounded in the theoretical frameworks of 'language teacher cognition' (Borg, 2003; 2006) and 'enhanced expertise' by de Jong, Harper, and Coady (2013), this study explores the associations among in-service teachers of ELLs' learning, cognition, and reported applications of L2 writing in their classrooms. Implementing a cross-sectional survey design, I surveyed 1,075 in-service teachers from the content areas of ESL, English language arts, social studies, math, science, and special education in New York. Data were analyzed with two standard multiple linear regression models. Results demonstrated a strong association between teacher learning and teacher perceived knowledge. Higher levels of teacher perceived knowledge are observed among female teachers holding an ESOL certification, knowing another language, and having received preparation for teaching ELLs and L2 writing. Furthermore, more varied reported applications of writing for ELLs were observed among ESOL certified teachers with increased cognition levels--that is, higher perceived knowledge of ELLs and L2 writing, more positive multicultural diversity attitudes, and beliefs finding the evidence-based writing practices significant--who work in a school implementing a comprehensive plan for teaching writing, and who have a higher number of ELLs with low levels of English language proficiency in their classrooms. The results have important implications for linguistically and culturally responsive teacher education--preparation for pre-service teachers and professional development for in-service teachers--and curriculum implementations to foster ELLs' writing skills. [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.]