Serotonin (5-hydroxytryptamine, or 5-HT) is a monoamine neurotransmitter synthesised in serotonergic neurons in the central nervous system (CNS) and enterochromaffin cells in the gastrointestinal tract of animals including humans. In the central nervous system, serotonin is believed to play an important role as a neurotransmitter, in the modulation of anger, aggression, body temperature, mood, sleep, sexuality, and appetite as well as stimulating vomiting. In addition, serotonin is also a peripheral signal mediator. For instance, serotonin is found extensively in the human gastrointestinal tract (about 90%), and the major storage place is platelets in the blood stream. As with all neurotransmitters, the effects of 5-HT on the human mood and state of mind and its role in consciousness are very difficult to ascertain. This new book presents the latest research in the field from around the world. The serotonin (5-HT) system is evolutionarily quite old and is found in all nervous systems. And although making comparative analysis provides ample opportunity for misinterpretation, sufficient similarities exist regarding the 5-HT system and its role in behavioural control to make some generalisations about the function of the 5-HT system and its relationship to specific behaviours and disorders. The 5-HT system is involved in the initiation or the inhibition of many behaviours in a "normal" behavioural repertoire, and many of those are of interest to psychologists. Such behaviours include, but are not limited to mood, aggression, impulsivity, sleep, appetite, and motor function. This book provides leading edge research on this field from around the globe.