The fragile X syndrome is the most frequent cause of inherited mental retardation. It is caused by a dynamic mutation: the progressive expansion of polymorphic (CGG)n trinucleotide repeats located in the promoter region of the FMRI gene at Xq27.3. The cloning of the FMRI gene and the elucidation of the molecular basis of the fragile X syndrome is of great importance for the diagnosis and understanding of this unusual type of mutation. Although extensively studied, the mechanism behind the transition from stable normal (CGG)n alleles to the carrier state (an unstable premutation) and from premutation to mutation is partially understood. The clinical diagnosis of fragile X mental retardation (FXMR) is not possible as dysmorphic features are subtle. Molecular diagnosis by Southern Blot is the confirmatory test that makes carrier detection and prenatal diagnosis possible. As the risk of recurrence of FXMR is high in the family and carrier relatives, an identification of fragile X positive children, and offering carrier detection and prenatal diagnosis to the families is very important. It is possible by screening mentally retarded children and adults even if there is no family history of mental retardation or typical behavioral or physical features associated with the fragile X phenotype. In this review we have discussed the method for the diagnosis and counseling of the families. The complexities due to premutation and the variable severity of manifestations in carrier females need to be understood while counseling fragile X families.