The forced oscillation technique (FOT) is a noninvasive method for measuring respiratory mechanics, offering advantages over conventional lung function tests by not requiring specific breathing maneuvers. This European Respiratory Society Task Force report outlines the basic principles, guidelines, and future developments of FOT for clinical use in both adults and children. FOT data, particularly at lower frequencies, are sensitive to airway obstruction but do not distinguish between obstructive and restrictive lung disorders. While there is no consensus on FOT's sensitivity for bronchodilation testing in adults, it is reliable for assessing bronchial hyperresponsiveness in both groups. FOT is as sensitive as spirometry in detecting lung function impairment due to smoking or occupational hazards and is ideal for epidemiological studies due to minimal patient cooperation requirements. New applications include monitoring respiratory mechanics during mechanical ventilation and sleep. FOT involves applying small-amplitude pressure oscillations during normal breathing, with measurements taken at the airway opening. The input impedance (Zrs,in) is calculated from the relationship between transrespiratory pressure and airflow. Zrs can be partitioned into pulmonary (ZL) and chest wall (Zw) components based on intraoesophageal pressure measurements. A head generator technique minimizes upper airway shunting by applying oscillations around the head. FOT is sensitive to respiratory mechanics across various frequencies, with lower frequencies reflecting spontaneous breathing and higher frequencies capturing inertial properties. The technique is useful for assessing airway reactivity, reversibility, and bronchial hyperresponsiveness. FOT parameters, such as Rrs and Xrs, are influenced by frequency and can be affected by upper airway shunting, which is minimized using the head generator technique. Reference values for Rrs and Xrs vary by age and sex, with higher values in females and younger individuals. FOT shows good reproducibility in both adults and children, with variability ranging from 5-15% in healthy adults. In children, day-to-day variability is higher, and circadian rhythms may affect Rrs. FOT is sensitive to airway obstruction and can detect changes in bronchial hyperresponsiveness, with a cut-off value of a 10% decrease in Rrs0 for significant reversibility. FOT is as sensitive as spirometry in detecting lung function impairment due to smoking or occupational hazards and is suitable for epidemiological studies. It is also useful in assessing bronchodilator responses and airway reactivity in children. However, FOT may not distinguish between intra- and extrapulmonary obstruction, and its sensitivity to airway stenosis is higher than spirometry. In cystic fibrosis, FOT may not correlate well with spirometric indices, and further studies are needed to clarify the mechanisms of impairment in respiratory function. Overall, FOT is a reliable and versatile technique for assessing respiratory mechanics in clinical practice.The forced oscillation technique (FOT) is a noninvasive method for measuring respiratory mechanics, offering advantages over conventional lung function tests by not requiring specific breathing maneuvers. This European Respiratory Society Task Force report outlines the basic principles, guidelines, and future developments of FOT for clinical use in both adults and children. FOT data, particularly at lower frequencies, are sensitive to airway obstruction but do not distinguish between obstructive and restrictive lung disorders. While there is no consensus on FOT's sensitivity for bronchodilation testing in adults, it is reliable for assessing bronchial hyperresponsiveness in both groups. FOT is as sensitive as spirometry in detecting lung function impairment due to smoking or occupational hazards and is ideal for epidemiological studies due to minimal patient cooperation requirements. New applications include monitoring respiratory mechanics during mechanical ventilation and sleep. FOT involves applying small-amplitude pressure oscillations during normal breathing, with measurements taken at the airway opening. The input impedance (Zrs,in) is calculated from the relationship between transrespiratory pressure and airflow. Zrs can be partitioned into pulmonary (ZL) and chest wall (Zw) components based on intraoesophageal pressure measurements. A head generator technique minimizes upper airway shunting by applying oscillations around the head. FOT is sensitive to respiratory mechanics across various frequencies, with lower frequencies reflecting spontaneous breathing and higher frequencies capturing inertial properties. The technique is useful for assessing airway reactivity, reversibility, and bronchial hyperresponsiveness. FOT parameters, such as Rrs and Xrs, are influenced by frequency and can be affected by upper airway shunting, which is minimized using the head generator technique. Reference values for Rrs and Xrs vary by age and sex, with higher values in females and younger individuals. FOT shows good reproducibility in both adults and children, with variability ranging from 5-15% in healthy adults. In children, day-to-day variability is higher, and circadian rhythms may affect Rrs. FOT is sensitive to airway obstruction and can detect changes in bronchial hyperresponsiveness, with a cut-off value of a 10% decrease in Rrs0 for significant reversibility. FOT is as sensitive as spirometry in detecting lung function impairment due to smoking or occupational hazards and is suitable for epidemiological studies. It is also useful in assessing bronchodilator responses and airway reactivity in children. However, FOT may not distinguish between intra- and extrapulmonary obstruction, and its sensitivity to airway stenosis is higher than spirometry. In cystic fibrosis, FOT may not correlate well with spirometric indices, and further studies are needed to clarify the mechanisms of impairment in respiratory function. Overall, FOT is a reliable and versatile technique for assessing respiratory mechanics in clinical practice.