Traditionally, the prevention of genetic diseases in patients who are at substantial risk of conceiving children affected by a known genetic defect has been achieved by prenatal diagnosis, using mainly chorionic villus sampling (CVS), amniocentesis and cordocentesis. This approach aims to establish the presence or absence of the disease to avoid the birth of affected children. The technologies developed for the Human Genome Project, the recent surge of available DNA sequences resulting from it and the increasing pace of gene discoveries and characterization have all contributed to new technical platforms that have enhanced the spectrum of disorders that can be diagnosed prenatally. DNA-based testing is becoming possible for an increasing number of hereditary diseases as the responsible genes are mapped to individual chromosomes and then isolated and characterized. The introduction of automated fluorescence-based DNA analysis methods provided a fast and accurate way for prenatal diagnosis of single gene disorders. The use of fluorescent PCR, direct sequencing and minisequencing techniques, has increased accuracy and reliability of the analysis, due to the higher sensitivity of the approach. Furthermore, automation and computerized analysis systems has increased the overall analysis throughput. Preimplantation genetic diagnosis (PGD) represents a very early form of prenatal diagnosis aimed at detecting genetic diseases on IVF embryos before their transfer into the uterus. PGD has revealed an acceptable alternative to prenatal diagnosis, mainly in those countries where pregnancy interruption is forbidden by law, or for couples where termination of pregnancy represents a religious and/or ethical problem. Using the above approaches, over 30.000 prenatal genetic tests and 250 PGD cycles have been performed in our department. The overall experience of using prenatal and preimplantation genetic diagnosis of single gene disorders presently exceeds 50 different diseases. Finally, the completion of the genome project combined with the recent advances in nanotechnologies, are enhancing the development of new diagnostic methods, such as microarrays (DNA chips), for rapid, accurate and simultaneous mutation detection. The next few decades hold the promise of many more advances in preimplantation and prenatal genetic testing.