Teachers have difficulty using data from Curriculum-based Measurement (CBM) progress graphs of students with learning difficulties for instructional decision-making. As a first step in unraveling those difficulties, we studied teachers' comprehension of CBM graphs. Using think-aloud methodology, we examined 23 teachers' ability to "read," "interpret," and "link CBM data to instruction" for fictitious graphs and their own students' graphs. Additionally, we examined whether graph literacy--measured with a self-report question and graph-reading skills test--affected graph comprehension. To provide a framework for understanding teachers' graph comprehension, we also collected data from "gold-standard" experts. Results revealed that teachers were reasonably proficient at "reading the data," but had more difficulty with "interpreting" and "linking the data to instruction." Graph literacy was related to some but not all aspects of teachers' CBM graph-comprehension ability. Implications for training teachers to comprehend and use CBM progress data for decision-making are discussed.