Small airways disease involves those airways that are less than 2 mm in diameter. These airways comprise a very large component of the cumulative cross-sectional area of the tracheobronchial tree. Thus, disease within small airways is often detected relatively late. In view of these and other complexities, a calibrated approach to diagnosis is required. This chapter discusses the various conditions that are capable of causing small airway disease, the clinical findings that should lead to an initial suspicion of the condition and the functional and CT-based assessment that leads to their eventual implication as a cause for dyspnoea.