The small airways of the lungs are commonly affected in pediatric and adult asthma. Small airways disease has been related to asthma control, severity, and risk of exacerbation. Diagnosis of small airways disease can be best made through evaluation of surgical lung specimens. Noninvasive techniques including spirometry, plethysmography, nitrogen washout, impulse oscillometry, and cross-sectional imaging have been utilized to assess and infer the extent of small airways disease in asthma and can be used longitudinally to assess response to treatment. Patients with small airways disease seem to benefit from inhaled asthma medications that have improved capacity to reach the distal lung compartment. This is especially important for patients with severe asthma, who rely upon high doses of inhaled corticosteroid and bronchodilators for asthma control. This review will describe the techniques which may be utilized to assess small airways disease, discuss the prevalence and characteristics of small airways disease in severe asthma, and highlight how small airway disease may complicate severe asthma treatment.