This chapter arose out of efforts to better understand the ways in which the content knowledge and subject matter background of elementary teacher candidates in a master’s certification program were consequential for their learning to attend to student ideas and reasoning in mathematics and science. During the program the authors, who were the teacher candidates’ mathematics and science methods course instructors, quickly noticed an interesting pattern: although the courses were designed with the same goals, similar activities, and parallel assignments, as the teacher candidates learned to attend to student thinking in science and mathematics, notable differences emerged in teacher candidates’ practices of attending to student thinking across the two disciplines. This chapter presents analyses and findings from a study aimed at explaining the important differences that became apparent across subject matters.