The occurrence of chromosomal abnormalities is a common theme in carcinogenesis. A large proportion of tumours which have been characterized at the cytogenetic level carries numerical and/or structural aberrations. Numerical alterations may include acquisition or loss of specific chromosomes or dramatic changes in overall ploidy levels. Structural aberrations may include DNA amplifications or deletions, inversions and translocations. Many chromosomal alterations occur in a non-random fashion and may be subdivided in to primary and secondary, according to their timing of occurrence. Primary chromosomal abnormalities usually occur at the early stages of tumourigenesis and are often encountered as sole cytogenetic abnormalities. Secondary chromosomal abnormalities are usually associated with more advanced stages of tumour development. In recent years several chromosomal abnormalities could be correlated with specific gene alterations, thus providing insights into the molecular mechanisms underlying tumourigenesis. The biological consequences imparted by other chromosomal changes such as numerical changes are, however, less clear. By using recently developed molecular techniques for chromosome characterization, so-called molecular cytogenetics, our perception on cancer cytogenetics is rapidly changing through the disclosure of hitherto unknown (specific) chromosomal abnormalities.