Chromosome abnormalities of cancer cells have been recognized for a long time, and have generally proven to be a highly specific marker ofmalignancy. The contri- butions collected in this book, Tumor Aneuploidy , cover several major aspects of present knowledge conceming the occurrence and clinical significance of chromo- some abnormalities as delineated by karyotype analyses or measurements of the cellular DNA content. Certain non-random clonal chromosome losses, deletions and translocations ap- pear to represent primary genetic lesions of malignancies and reflect their clonal origin. Secondary intraneoplastic genetic evolution is suggested by major clonal ab- normalities of chromosome number and cellular DNA content. Both types of ge- netic changes have been reaching great relevance in cancer medicine, today. Although the Philadelphia chromosome was first discovered in chronic myelo- cytic leukemia (CML), by Nowell and Hungerford in 1960, new banding techniques developed in the 1970’s were needed to identity this abnormality as a translocation between chromosomes 9 and 22 (t(9; 22)). Soon thereafter, further non-random translocations were detected and attributed to special diseases like t(8; 21) and t(15; 17) to acute myeloid leukemias (AML) and t(9; 22), t(4; 11), t(8; 14) to acute lymphoblastic leukemia (ALL).