The present book is an in-depth synopsis of recent advances in the fragile X field. The fragile X syndrome (FXS) is the most common form of inherited mental retardation, and the molecular-genetic basis of this syndrome has been the subject of extensive experimental scrutiny over the last two decades. This book covers the spectrum of current FXS research, ranging from Drosophila model systems via mouse models to clinical and psychiatric aspects. The volume also provides updates on FXS-related diseases such as the fragile X-associated tremor/ataxia syndrome. Significant progress in recent years, as highlighted here by some of the most eminent researchers in the field, are grounds for optimism that successful therapeutical approaches may be feasible in the not too distant future.