Many athletes incorporate high training volumes and limited recovery periods into their training regimes. This may disrupt the fragile balance and the accumulation of exercise stress may exceed an athlete's finite capacity of resistance. A state of elevated fatigue, increased mood disturbance and decreased exercise performance can result. This is commonly known as overreaching and if increased training and limited recovery is continued, it is believed that the more serious state of overtraining may develop. This is relatively commonly experienced in athletes, however little scientific investigation has been conducted to determine the characteristics and underlying mechanisms. The overall aim of this thesis was to gain a greater understanding of the state of overreaching and to specifically provide new information on potential markers of this state as well as possible mechanisms. To study the cumulative effects of exercise stress and subsequent recovery on performance changes, fatigue indicators and possible mechanisms, the training of endurance cyclists was systematically controlled and monitored in two separate investigations. A number of variables were assessed including performance, physiological, biochemical, psychological, immunological and hormonal variables. In addition heart rate variability and serotonergic responsiveness were also assessed. Some of the more pertinent effects of overreaching included an increase in heart rate variability, a reduction in carbohydrate oxidation, an increase in serotonergic responsiveness and a reduction in stress hormone concentrations. These results suggest that autonomic imbalance in combination with decreased hormonal release appears to be related to the decline in performance and elevated fatigue apparent in overreached athletes. Additionally it also appears that alterations in the hypothalamic-pituitary adrenal axis may occur in overreached athletes.