Formative assessment practices, including eliciting a broad range of student ideas, noticing the nuances in students' ideas, using these ideas to guide instruction, and promoting student self-regulation of learning are key components of expert teaching. Given the inherent dialogical nature of formative assessment in the classroom, video can provide a powerful tool for capturing and analyzing teachers' formative assessment interactions with students. In this study, we provide a framework for examining expertise in formative assessment and use this framework to quantitatively and qualitatively analyze the practices of 13 mathematics and science teachers. While we only saw a few instances of true expertise in formative assessment practices in our examination of videos, our findings indicate that teachers with more expertise in formative assessment let students' ideas guide their teaching. This leads to higher correlations among the dimensions of practice that we articulate in our framework for expert teachers. However, because many of the instructional decisions that teachers make are not visible on video, video alone may not provide enough information to judge expertise in formative assessment.