Ms. Carlisi teaches middle school mathematics for students who were retained at least 1 year and who were identified as at risk of failure in mathematics. About half of her class of 14 students has identified learning disabilities and/or emotional and behavioral difficulties. Ms. Carlisi worries constantly that she is not going to be able to help them "catch up" by the time they take the mandated state high-stakes standardized assessment. She decides to use Mathematics Dynamic Assessment (MDA) to help her students. MDA is an informal mathematics assessment process that integrates four research-supported assessment practices for struggling learners: (1) Assessment of students' interests and experiences; (2) Concrete-representational-abstract assessment within authentic contexts; (3) Error pattern analyses; and (4) Flexible interviews. The MDA process provides teachers with important information about what students do and do not understand about foundational mathematics concepts, students' levels of understanding and abilities to express their understandings, and where students are in the learning sequence (frustration, instructional, mastery). The data collected through the MDA process provide teachers with an in-depth evaluation of their students' mathematical understandings and thinking that allows teachers to plan their instruction to address students' specific mathematical learning needs. (Contains 10 figures and 1 table.)