<b><i>Background:</i></b> Measurement of respiratory muscle function is important in the diagnosis of respiratory muscle disease, respiratory failure, to assess the impact of chronic diseases, and/or to evaluate respiratory muscle function after treatment. <b><i>Objectives:</i></b> To establish reference values for maximal inspiratory and expiratory pressure, and the tension-time index at rest in healthy children and adolescents aged 8–19 years, as well as to present sex- and age-related reference centiles normalized for demographic and anthropometric determinants. <b><i>Methods:</i></b> In this cross-sectional observational study, demographic, anthropometric, and spirometric data were assessed, as well as data on respiratory muscle strength (P_Imax and P_Emax) and work of breathing at rest (T_T0.1), in a total of 251 children (117 boys and 134 girls; mean age 13.4 ± 2.9 years). Reference values are presented as reference centiles developed by use of the lambda, mu, sigma method. <b><i>Results:</i></b> Boys had significantly higher P_Imax and P_Emax values. Next to sex and age, fat-free mass appeared to be an important predictor of respiratory muscle strength. Reference centiles demonstrated a slight, almost linear increase in P_Imax with age in boys, and a less steep increase with age in girls. T_T0.1 values did not differ between boys and girls and decreased linearly with age. <b><i>Conclusion:</i></b> This study provides reference values for respiratory muscle strength and work of breathing at rest. In addition to sex and age, fat-free mass was found to be an important predictor of respiratory muscle strength in boys and girls.