Conceptual accounts of good practice in listening pedagogy and assessment have emphasised the value of a ‘diagnostic’ approach (Field, 2008; Harding et al., 2015). Broadly, such approaches focus on methods and tasks designed for problem identification and problem solution. However, views differ on precisely what a diagnostic approach might entail, and there remains little empirical research to support the effectiveness of various diagnostic procedures for improving listening. This chapter provides an overview of the topic. We first introduce, define and problematise the concept of “diagnosis” in the field of second language teaching and assessment. We then provide an overview of critical issues, focusing first on the types of diagnostic approaches advocated in the second language listening classroom, and second on different approaches to diagnostic listening assessment, considering both purpose-built diagnostic tests and the generation of post-hoc diagnostic evidence through cognitive diagnostic modelling. Based on this, we provide some recommendations for practice both for teachers and assessors, arguing that a diagnostic approach naturally and productively blurs the boundaries between good teaching practice and good assessment practice. Finally, we discuss future directions for diagnostic approaches in teaching and assessing listening with a specific focus on the limitations and affordances of new technology.