Recent research and policy documents call for engaging students and teachers in scientific practices such that the goal of science education shifts from students "knowing" scientific and epistemic ideas, to students "developing and using" these understandings as tools to make sense of the world. This perspective pushes students to move beyond the rote performance of scientific actions or processes and engage instead in purposeful knowledge construction work. This raises parallel questions about how to go beyond characterizing student performance of scientific process to understand their engagement in scientific practices as a goal-directed activity. To that end, this article offers a framework--the "Epistemologies in Practice" (EIP) framework--for characterizing how students can engage meaningfully in scientific practices. This framework emphasizes two aspects of student engagement in scientific practices: (1) the students' epistemic goals for their knowledge construction work and (2) their epistemic understandings of how to engage in that work.