This booklet is divided into seven sections that include the following topics: (a) physical performance, (b) adaptation to inactivity and activity, (c) physiological and medical motives for regular physical activity, (d) training, (e) physical fitness for everyday life, and (f) testing physical fitness and condition. Section one discusses energy output, neuro-muscular function, psychological factors, and age and sex. Section two discusses the changes in our bodies in adjusting to inactivity. Section three discusses cardio-vascular diseases, obesity, diet and choice of calories, hot environment and water balance, diseases and troubles of the back, joints, and posture. Section four includes the four principles of training, and a discussion of a simple training ground, and equipment. Section five discusses active recreation. Section six describes how the bicycle ergometer measures performed external work. The summary describes the responsibility of government and society for creating recreational areas and lists five points to remember. The author concludes that neglect of regular physical activity during adolescence cannot fully be compensated for later in life and that young people should acquire knowledge and experience of activities suitable for their future in school. These activities should include running, cycling, swimming, and skiing rather than the more organized team sports. A 49-item bibliography is included. (PD)