The author presents and discusses the problems of work physiology in the age of development and changes of physical performance during the second decade of life on the basis of the results of her own cross-sectional and two series of longitudinal studies. The main considered problems are as follows: Morphological, physiological and psychological determination of adaptation of children and adolescents to physical exercises. The level of aerobic and anaerobic capacity of Warsaw children aged 9-15 years. Cross-sectional study (n = 667); measurements: PWC170 using bicycle ergometer and step test, anaerobic capacity using Wingate Anaerobic Test according to Bar-Or (1978). Dynamics of changes of physical work capacity (PWC170) in children and adolescents 11-17 years old in relation to their physical activity, chronological and biological age--longitudinal studies (n = 112). Dynamics of changes of aerobic capacity (VO2 max--directly measured) in children aged 11-14 years in relation to their chronological, biological age and sport disciplines--longitudinal studies (n = 94). Motor development and reaction to different types of exercises in obese 7-15 years old children. Cross-sectional studies (n = 257); measurements: physical fitness using Denisiuk's, Chromiński's, Ozjerecki's tests; tapping test; some indices of respiratory function; PWC170; haemodynamic reaction on isometric exercise; physical activity and work load during physical education lessons. In conclusion about every above mentioned chapter practical indications for physical education, sports and health care are recommended. The elaborated norms for some indices of aerobic and anaerobic capacity may be useful in individual diagnosis of physical work capacity of healthy and sick children, monitoring its changes during growth as well as in observation of secular changes of physical performance in the future.