Mathematics assessment is integral in enabling teachers to implement specially designed instruction for students with disabilities. This article presents an overview of an underutilized form of assessment, the clinical interview. A type of diagnostic assessment, clinical interviews are critically important when designing individualized instruction because the assessment provides information about how a student understands something or why a student is struggling. A rationale for its use is provided along with step-by-step instructions on how to implement a clinical interview to effectively document students' strengths and weaknesses. The article concludes with an example of how a clinical interview can be used to directly inform instruction.