Mathematics teachers want to empower students as mathematical thinkers and doers (NCTM 2000). Specific ways of thinking and doing mathematics were described in the Process Standards (NCTM 2000); they were further characterized as habits of mind (Mark, Goldenberg, and Sword 2010); and more recently, they were detailed in the Common Core's Standards for Mathematical Practice (CCSSI 2010). Mathematical practices (or MPs) are essential practices that students use to make sense of concepts and procedures. Through formative assessment, students' work can be used to help them become aware of MPs and improve their capacity to think and behave like mathematicians: that is, to use mathematical tools to explore situations and solve problems, to uncover and describe the structure of relationships, and to create and evaluate mathematical arguments. In this article, the authors describe and illustrate a formative assessment process to improve students' proficiencies in the MPs and suggest differentiated feedback to support learning progressions for MPs.