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Mental training has been studied extensively for the past century but we are still not completely sure how it affects brain and behavior. The aim of this doctoral thesis was to examine one aspect of mental training i.e. motor imagery. In Study I, active high jumpers were trained for 6 weeks using a motor imagery mental training program. We measured behavioral effects in motor parameters such as total height, false attempts, take off angle, and bar clearance. A significant improvement was found on the bar clearance component compared to a control group of high jumpers that did not participate in the mental training program. The results emphasize the importance of using appropriate outcome measures since mental training may affect distinct features of the movement rather than the entire movement. Study II used fMRI (functional Magnetic Resonance Imaging) to examine the neural correlates of imagery for active high jumpers, and also how imagery training affects brain activity. Active high jumpers were compared to a control group of high jumping novices and the results showed that high jumpers were able to activate motor regions, whereas controls used parts of the visual system to perform imagery of the high jump. Thus, we were able to show how important well established motor representations are in order to achieve a neural overlap between imagery and action. In study III we examined the effects after motor, mental and combined motor and mental training on a finger tapping task. Behaviorally, even though mental training improved performance, adding mental training to motor training did not improve the results beyond only using motor training. Imaging results showed that motor and mental training engaged different neural systems, with motor training associated with motor activity and mental training with visual activity. The combination of motor and mental training activated both motor and visual systems. Additionally combining motor and mental training resulted in transfer to an untrained motor sequence and neural data indicated that cerebellum mediated the transfer. The overall findings explain how mental training can be used to improve motor performance and motor parameters. Moreover, it also illustrates that the neural processes underlying such improvements may be distinct from motor training and that the brain may react differently during mental training depending on prior physical experience of the action.