Lung volume measurements always determine total lung capacity (TLC), and most measurement techniques also determine functional residual capacity (FRC) and residual volume (RV). These measurements are needed less often than spirometry-derived values and require somewhat more difficult methods than required by spirometry. As with spirometry, accurate interpretation relies on appropriate reference values, standardized methodology and technically well-performed testing. A reduction in TLC defines chest restriction. Increases in TLC, FRC and RV are often seen in patients with airway obstruction. Patients with mixed restrictive and obstructive disease may have spirometry results that are difficult to interpret, and it is in these patients that lung volume measurements may be especially helpful.