The ultimate public health aim of genetic screening is prevention. This can be achieved by reducing birth prevalence through primary or secondary methods such as pre-conceptional or antenatal screening. Tertiary prevention by neonatal screening is also an option where there is direct unbiased evidence for a substantial improvement in prognosis. In addition to this, the information provided during screening is also of value, enabling individuals to make choices that otherwise would not have been available. Having elucidated the natural histories and genetic defects underlying two common, serious genetic disorders, cystic fibrosis and fragile X syndrome, considerable efforts have been channelled into ascertaining the most efficacious method of prevention. To date there is only indirect evidence to suggest that neonatal screening improves prognosis in cystic fibrosis. Similarly, treatment for fragile X syndrome is limited and therefore early identification of the disorder by neonatal screening is unlikely to improve long term outlook. Thus the focus of this review is on primary and secondary preventive methods.