Peripheral hormones have a major impact on the brain: they are able to interfere with its development, to affect release of neurotransmitters and concentrations of receptors, to trigger growth factors involved in lesion repair. These multiple actions account for their capacity to modulate a number of physiological parameters, from reproductive functions to memory, behavior and aging. This book based on contributions of pioneer investigators in the field, outlines the role of hormones in pathogenic processes such as mental disturbances or neurodegenerative diseases.