Teachers make many important decisions about instruction during the process of planning. While educational researchers have extensively explored teacher planning, this research has largely neglected the ways teachers attend to subject matter while planning. We first explore ways a sample of 13 experienced secondary biology teachers attend to science concepts in planning. Teachers in this study reported attending to science ideas in order to decide what was important to teach and to enhance their own knowledge of science concepts. We next introduced teachers to a tool, called the concept model, that is designed to support teachers in attending to science concepts while planning. After completing a concept model, participants reported that the tool influenced them to focus on important aspects of the concept, recognize the range of complexity in the topic, consider aspects students find difficult, evaluate their own understanding, and identify connected concepts. These findings extend our understanding of how teachers attend to subject matter during planning and suggest the potential of a tool like the concept model for supporting that process.