In sports training sessions and competitions we often come across the phenomena of hyperthermia and dehydration which cause physical fatigue and interfere in achieving high sports results. It is not clear, what is the effect of pre-exercise rehydration to central fatigue under conditions of hyperthermia. The main purpose of the study was to establish the impact of hyperthermia and dehydration on the skeletal muscle fatigue of men and women both actively engaged and not engaged in sports when performing physical exercises of high intensity. I also aimed to analyze the impact of pre-exercise rehydration on central fatigue under the conditions of hyperthermia performing isometric exercises of maximum intensity. The most important results from experimental-clinical research, obtained from my doctoral dissertation, are described and analyzed in this book. All the research was done in Lithuanian Academy of Physical Education, the Laboratory of Human Motor Control. I am very grateful for all the laboratory employees and research participants for helping me with this research.