Children aged 3~5 years old represent the challenge in pulmonary function assessment, since evaluating lung function in preschool age group is important for the appropriate treatment for patient with chronic and recurrent cough and wheeze during this period. The joint American Thoracic Society/European Respiratory Society task force has produced recommendations for the spirometric lung function test currently used in the preschool age group. The reliable scientific evidence, documented references and reviews by the experts were used as a support. Reference data of spirometry lung function in preschool children were available in several countries including USA, Norway, Czech, Israel, Canada, and Taiwan. Spirometric pulmonary function tests are feasible in 3~ to 5~year~old children. However, the existing data are not sufficient to make definitive recommendations. Recommendations will need to be revised periodically until sufficient evidence has been collected to make definitive guidelines in various situations.