Research in mathematics education increasingly recognizes the role of language in the education of English language learners. However, research examining the professional growth of mathematics teachers as they learn to teach English language learners is sparse. This case study addresses this issue by examining one third grade teacher as she learned how to focus on language as she designed and taught mathematics lessons to facilitate the participation of English language learners. Data sources consist of audio recordings of interviews, lesson planning sessions, and lesson debrief sessions as well as video recordings of mathematics lessons. The results from this study demonstrate the importance of professional development that emphasizes language in the teaching of mathematics. As the teacher began to learn about the components of the intervention, she developed specialized knowledge and competencies so that she was able to address language planning and development that was necessary to successfully teach mathematics to English language learners.