This case study investigates a professional development initiative in which a first-grade bilingual teacher engages in learning and teaching Cognitively Guided Instruction, a framework for understanding student thinking through context-rich word-problem lessons. The study explores (a) the impact of classroom-based professional development on a teacher's understanding of teaching mathematics to Latina/o students, and (b) issues of language and culture with which the teacher grapples while engaged in reflecting on students' mathematical thinking. Our findings show that ongoing reflection, collegial conversations, and analysis of students' work enhanced teacher's understandings of students' mathematical learning, and of practices that provide students opportunities to solve contextualized mathematics problems, to communicate their solutions, and to represent their thinking. (Contains 4 footnotes and 1 table.)