Since the establishment of human karyotype in 1956, human cytogenetic has quickly progressed. The description of the Philadelphia chromosome in 1960 led up to new applications of cytogenetic in the fields of hematology and oncology. The initial techniques allowed only uniform staining of chromosomes, limiting the detection of most structural rearrangements. Many approaches aimed to gain a better knowledge of chromosomal structure, a better understanding of rearrangements, and a better identification of the chromosomes were developed: autoradiography, banding techniques, electronic microscopy. Since 1980, new developments in clinical cytogenetic and molecular biology have occurred. In situ labeling using non-radioactive probes onto chromosomes and nuclei was developed: fluorescence in situ hybridization (Fish) was born. Fish allows detecting many chromosomal abnormalities of number and/or structure. The major limitation of this technique is that its use should be based on known indications for the choice of the probe. Multicolor karyotype (M-Fish or Sky), the most recent development of Fish on metaphase spreads, allows to overcome this limit. As shown here in three examples, M-Fish allows to describe precisely complex rearrangements in hematological malignancies and solid tumors. Finally, if no metaphase is available, comparative genomic hybridization (CGH) can be performed to detect and simultaneously localize on chromosomes gains or losses in genomic DNA.