PURPOSE OF REVIEW: To highlight the recent evidence of the lung function techniques used in preschool children to diagnose asthma. RECENT FINDINGS: Several techniques are available to measure lung function and airway inflammation in preschool children, including spirometry (from age 5 years), impulse oscillometry (>3 years), whole-body plethysmography (>3 years), fractional exhaled nitric oxide (FeNO) (>5 years), multiple breath washout (>3 years), structured light plethysmography (>1-2 years) and impedance pneumography (>1 years). If applicable, measuring forced expiratory volume in 1 s (FEV1) and FEV1/forced vital capacity (FVC) ratio using spirometry is useful (cut-off < 80% predicted or below lower limit of normal [LLN] defined as z-score < -1.64) for diagnosing preschool asthma. For those unable to perform spirometry, whole-body plethysmography (sRaw > 1.6 kPa/s) and impulse oscillometry (Rrs and Xrs at 5 Hz z-score > 2) may be useful. Adding a bronchodilator reversibility test (FEV1 increase > 12%, sRaw decrease > 25-30%, Rrs at 5 Hz decrease > 40%) or a bronchial challenge test, for example, exercise test (FEV1 decrease > 10%), may improve the sensitivity of these tests. Elevated FeNO (>25-35 ppb) is a promising adjunctive test for diagnosing preschool asthma. SUMMARY: With trained personnel, lung function testing can be done with high reliability even in children between 2 and 4 years of age. To avoid over and undertreatment of asthma, objective measurement of lung function is clinically important in preschool children.