In contrast to spirometry, airflow resistance determinations provide an effort independent measure of the airway status and allow measurement in individuals unable or unwilling to provide adequate effort during spirometry. Resistance measurements may be performed using an esophageal balloon, airflow perturbation techniques (including interrupter and oscillatory techniques), or total-body plethysmography. Esophageal balloons are invasive, and airflow perturbation techniques are becoming more widely used. Airflow perturbation methods assess small airway dysfunction using frequency dependence of resistance, a surrogate for frequency dependence of compliance. Body plethysmography remains the “gold standard” for measuring airway resistance and is based on measures of pressure changes and flows with the patient enclosed in a body plethysmograph. While plethysmographic procedures may be completed rapidly, yielding multiple trials within preset repeatability criteria, the equipment is costly and the operator must be highly trained. Plethysmographic specific resistance loops have definite shapes (morphologies) indicative of specific airway disorders, which may be interpreted in a manner analogous to spirometry. Specific resistance and conductance assess the important effects of lung volumes. Reimbursement for resistance measurements varies depending on regional guidelines.