Mapping of the human genome has been a global effort utilizing both genetic and physical mapping techniques. One approach which has greatly facilitated the physical mapping of the human genome is fluorescence in situ hybridization (FISH). Although FISH is by now a well-established technology, new recently developed modifications have enabled an easier use and higher resolution. The high-resolution FISH techniques have given a special impact in positional cloning: searching the functional gene from a chromosomal area where the gene has been genetically localized. New high-resolution FISH techniques include hybridization of probes to free chromatin, DNA fibres or mechanically stretched chromosomes. These targets have widened the resolution of FISH to detect distances from the traditional cytogenetic resolution level down to a resolution of a few kilobases. They also have significantly speeded up high-resolution physical mapping and thus made the search of new disease genes easier.