Over the past few decades the involvement of neurosteroids in brain function and mental health has attracted much interest, not only from a neuroscience perspective, but also from clinical and therapeutic aspects. In recent years, much of the research has focused on the multifunctional position of neurosteroids in the nervous system. The areas that have been explored, in particular, are the cellular and molecular mechanisms involved in the activity of neurosteroids in the brain, the role of neurosteroids in brain development, neurodegeneration and neuroplasticity, and their pharmacological properties. The large increase in the number of publications highlights this amplified interest in neurosteroids. Over the last 7 years at least 400 papers have been published describing the putative role of neurosteroids in the modulation of basic brain function and in the etiology and treatment of psychiatric and neurological disorders. Contributors to this book are amongst the most active basic researchers and c- nicians in the field and provide new perspectives not only clarifying some of the ongoing controversies, but also proposing diverse aspects and new insights. The book is organized in two major sections: “Neuroactive steroids and brain function’, and ‘Neuroactive steroids and neuropsychiatric disorders”. Though the selection of topics has been influenced by the current state of the art and the issues that require further elaboration, one should not assume that these are the only significant issues at this time.