An increasing number of researchers are calling for learning progressions to be used as interpretive frameworks for teachers conducting classroom assessment. The argument posits that by linking classroom assessments to learning progressions, teachers will have better resources to interpret and take instructional action on the basis of what students know. In this paper, we draw on data from a research project in which we have supported high school biology teachers in interpreting student responses to a multiple-choice classroom assessment linked to a learning progression for natural selection. We draw upon multiple sources of data, including student responses to a pre-post classroom assessment, artifacts from professional development sessions, and videotapes and transcripts of professional development meetings to construct a case study of one department of teachers as they came to understand the learning progression, interpreted results of classroom assessments, and revised their instruction. We use this case to illustrate the promise and challenges associated with linking classroom assessments to learning progressions. We conclude with recommendations to the field and suggestions for future work in this area.