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, behaviour and aging. Depending upon intensity and duration of exposure, they can be either neuroprotective or neurotoxic, for instance by affecting production of free radicals. This book, based on contributions of pioneer investigators in the field, outlines the ambiguous actions of gonadal steroids (estradiol, progesterone, testosterone, inhibin and activin) and of neurosteroids, related moieties produced in the brain itself. After summarizing their multiple mechanisms of action, which involve both direct effects on neuronal membranes and activation of genes coding for specific proteins in neurons or glial cells, the book outlines the role of hormones in pathogenic processes such as mental disturbances or neurodegenerative diseases.