After characterizing and giving the reasons for the study of this subject, the author goes on to give the results of mutual relationships between individual components of child development, namely between children’s physical efficiency, growth, physical development and some aspects of the psychical development. It has been found that the rate of development growth on the one hand and the rate of growth of physical efficiency both slow down with nine-year-old girls and ten-year-old boys. This is probably caused by the so-called growth pause, i. e. the dividing line between two growth periods (the end of the second childhood and the onset of pre-pubertal acceleration). A comparison between the increases in physical development and physical efficiency has shown that the rate of physical development growth was lower than the rate of physical efficiency growth. But the physical development of the pupils was more even than the efficiency development. A relatively parallel physical and efficiency development followed from the beginning to the end of the research has been found only in a minority of children: 12 per cent of the boys and 23 per cent of the girls of the sample concerned. A mildly significant relationship between physical development and efficiency development has only been found in seven- -year-old girls. The study of the children’s efficiency with regard to their bodily constitution has shown that an excessive weight of the body in relation to its height was not advantageous for the growth of overall physical efficiency. Those who had the best pre-requisites for efficiency growth were the harmonious, medium types of children and then those of the slim type. Only weak positive mutual relationships have been found between the somatic, the motor, and the psychical development. With the growing age some of these positive correlations weakened even further so that with nine and ten-year-olds it was not possible to prove them, and consequently it was not possible to judge from one quality what the other qualities would be like. Besides these results of the study of relationships between individual components of children’s development, the author gives conclusions drawn from a five-year longitudinal research. These conclusions are drawn from the study of children’s growth and physical development, of the development of children’s physical efficiency. Demands are put forward for an up-to-date controlled P. T. process, which influences the physical efficiency of the children, and some recommendations are made for further research into the subject dealt with.