Preschool children have a short concentration span and relatively poor cooperation with pulmonary function tests (PFTs). This combination complicates the inhalation protocol and the assessment of bronchodilation/bronchoconstriction in this age group. Bronchial responsiveness can be studied after bronchodilator administration or during bronchial challenge using miscellaneous PFTs. This article enclosed information on when to consider significant a PFT change, and published proposed cutoffs for bronchodilation/bronchoconstriction test. Issues on interpretation of bronchial test are also discussed. Recommendations are provided on behalf of the Interrupter Technique Subcommittee of the ATS/ERS Working Group on Infant and Young Children Pulmonary Function Testing.