Abstract
The paper presents the phenomenon of fatigue as an inevitable and insufficiently studied notion in the course of training. Besides the most famous and most used theories that explain this phenomenon, the factors leading to fatigue, methods and techniques which determine the existence of fatigue at various levels of human functioning were discussed. By analyzing the information sources an insight was obtained on the level achieved in the research of this phenomenon. Most of the levels at which it occurs were identified in the researches. However, it was underlined that there are numerous possible fatigue factors that further complicate projecting of one perfect model, that could be used to interpret that phenomenon. Contrary, the analysis showed that it is safer to consider the factors that lead to fatigue. Finally the analysis contributed to identification of further directions of monitoring and research of this phenomenon.