The completion of the human genome project enables us to understand the molecular pathophysiology of human genetic diseases more deeply. In addition, information from genomics has been utilized for the diagnosis of genetic disorders. The technological innovations have been explosive in the field of genetic testing. However, overwhelming genetic information often misleads physicians as well as patients to a wrong belief in the power of genetic testing. Genetic testing implicates many issues such as ethical, legal and social issues. Diagnostic genetic tests can be divided into three primary but overlapping categories: cytogenetic studies (including routine karyotyping, high-resolution karyotyping, and fluorescent in situ hybridization (FISH) studies], biochemical tests, and DNA-based diagnostic tests. DNA-based testing has grown rapidly over the last decade and includes pre- and postnatal genetic testing for the diagnosis of Mendelian diseases in patients, carrier testing, the determination of individual susceptibility to common complex diseases, and population screening of common genetic diseases in a particular population. Theoretically, once the structure, function, and disease association of a gene are well clarified, the clinical application of the genetic testing seems to be feasible. However, the test has to satisfy certain criteria for clinical application at a routine clinical setting; a high sensitivity and positive predictive values, the availability of a controllable quality assurance program, the determination of whether the test is replacing or is complementary to the traditional test, a cost-benefit issue, the possibility of treatment or disease-course modification, possible pre-and postnatal genetic counseling, and so on. In the near future, the application of genetic testing will be further expanded to common diseases and the pharmacogenetic assessment of individuals.