Diagnostic assessment (DA) in language classrooms is reported to have a substantial role in providing immediate and quality feedback for remedial work that would improve learning (Alderson, Brunfaut & Harding; 2014; Hattie & Timperley, 2007; Huhta, 2008; Jang & Wagner, 2013). However, applying DA in class remains a challenge for many practitioners for at least two reasons: First, there is no clear conceptualization of DA in the field of second language acquisition (Alderson, 2005; Alderson et al., 2015; Knoch & Macqueen, 2017). Despite advancements in teacher education (TE) programs, they mostly provide only a handful of courses to equip teacher candidates with necessary classroom assessment knowledge (Popham, 2011). To address the issue, this study attempted to explore DA practices and procedures of experienced university EFL instructors with the hope of developing a sound and empirically supported framework for classroom-based DA in the EFL context. The data came from 17 instructors with an MA in TEFL and related fields with more than 5 years of teaching experience. They received scenarios presenting certain problems, asked to explain what procedures and processes they would employ to diagnose the problem, and what they would do as remedial instruction. Based on the findings, we propose a data-driven framework for classroom-based DA. The paper also provides a discussion of the need to incorporate theoretical and practical dimensions of DA in pre-service and in-service TE programs.