X-linked mental retardation (XLMR) is a most exciting field of modern medical genetics. It made spectacular advances over the last twenty years, after the advent of molecular genetics. The discovery of the FMR1 gene unraveled the cause of the most common form of heritable mental retardation and provided the prototype of dynamic mutations. New genes continue to be mapped to the X chromosome and more and more are being cloned and characterized, clarifying the nosology of XLMR and, more importantly, adding to our understanding of the mechanisms of intellectual development, normal and abnormal. Looking back to a more or less recent past may provide clues for future discoveries.