The serotonergic and circadian systems constitute two of the main regulatory signaling networks of the brain. Each of them consists of a highly localized neuronal population that exerts a widespread influence on multiple cognitive, behavioral, physiological and molecular functions. In this book, the authors discuss the evolution, functions and roles in disease of the serotonergic system. Topics include interactions of the serotonergic and circadian systems in health and disease; serotonin’s disease-related changes and common clinical disorders; the serotonergic component in Parkinson’s disease symptoms and therapy side-effects; co-localization of serotonin and other classical neurotransmitters within the same neuron; and serotonin as a key neurotransmitter in suicide.