The new techniques of molecular cytogenetics, mainly fluorescence in situ hybridization (FISH) of DNA probes to metaphase chromosomes or interphase nuclei, have been developed in the past two decades. Many FISH techniques have been implemented for diagnostic services, whereas some others are mainly used for investigational purposes. Several hundreds of FISH probes and hybridization kits are now commercially available, and the list is growing rapidly. FISH has been widely used as a powerful diagnostic tool in many areas of medicine including pediatrics, medical genetics, maternal–fetal medicine, reproductive medicine, pathology, hematology, and oncology. Frequently, a physician may be puzzled by the variety of FISH techniques and wonder what test to order. It is not uncommon that a sample is referred to a laboratory for FISH without indicating a specific test. On the other hand, a cytogeneticist or a technologist in a laboratory needs, from case to case, to determine which procedure to perform and which probe to use for an informative result. To obtain the best results, one must use the right DNA probes and have reliable prools and measures of quality assurance in place. Also, one must have sufficient knowledge in both traditional and molecular cytogenetics, as well as the particular areas of medicine for which the test is used in order to appropriately interpret the FISH results, and to correlate them with clinical diagnosis, treatment, and prognosis.