This qualitative study explores K-12 classroom teachers' perceptions and experiences in meeting the needs of language learners, specifically English language learners (ELLs) and dual-language learners (DLLs). Addressing two main research questions, it investigated how teachers' preservice learning and development prepared them for instructing language learners and identified preparatory experiences and professional learning considered beneficial and critical to their development in this area. Through surveys, teacher interviews, and classroom observations, the study highlighted challenges faced by teachers in settings with diverse language populations, including native Japanese, Spanish, and Arabic speakers, as well as native English speakers learning a second language. It exposed gaps in teacher preparation programs, emphasizing deficiencies in coursework, experiences, and fieldwork related to methods and strategies for language learners. The study highlighted the importance of creating supportive and collaborative classroom and school environments for mainstream teachers and ELLs, and advocating for increased professional development opportunities tailored to mainstream teachers' instructional knowledge. Recommendations included revising teacher education programs to ensure comprehensive preparation for working with language learners and implementing induction or mentoring programs to guide inexperienced teachers in meeting these students' needs. Ongoing professional development aligned with teachers' needs and the school's student population is deemed essential for effectively catering to the needs of language learners. [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.]