In tumor cytogenetics, individual cancer cells are genetically analyzed, either conventionally by studying the mitotic chromosomes of dividing tumor cells from in vitro cultures (metaphase analysis; karyotyping) or molecularly by identifying specific DNA sequences or chromosomal regions in the nucleus of nondividing tumor cells in situ (interphase fluorescence in situ hybridization). In a typical cytogenetic study, 20 metaphase cells and/or 200 interphase cells are microscopically investigated for visible somatic chromosome changes that might occur in a given neoplastic clone. The detection sensitivity for aberrant cells, therefrom, ranges roughly between 1 and 10% in both, conventional and molecular cytogenetics. This and the need for a culturable testing material is why cytogenetics is completely different to the more sensitive molecular genetic methods (polymerase chain reaction, deep sequencing). The great strength of cytogenetics is the acquisition of substantial amounts of information about the whole genome of a tumor, cell per cell, in a single assay, such as detection of previously unknown chromosome anomalies, revealing of primary and secondary changes, proof of clonality and cell lineage origin, positional evidence for tumor-relevant genes, knowledge of the proliferating ability of an aberrant clone, and conclusions for staging and prognosis at diagnosis, as well as information about disease regression and clonal evolution during follow-up.