Malignant tumors of soft tissue constitute a very heterogeneous group of tumors which is composed of more than 50 different entities derived from either the mesenchyme or the neuroectoderm. Diagnostic problems can be linked to the difficulty to firmly establish the diagnosis of malignancy and to the precise determination of the type of sarcoma. The prognostic is largely dependent on local and distant extension of the cancer. The study of the genotype of tumor cells has enabled the detection and characterization of genetic alterations. These alterations, frequently chromosome translocations, are specific markers for subgroups of tumors and can thus be of clinical relevance. These markers can be detected by cytogenetic, in situ hybridization or molecular biology techniques. The specificity of these alterations for tumor cells and the possibility to detect them with the highly sensitive polymerase chain reaction (PCR) technique have enabled development of tests aimed at the detection of tumor cells within potential metastatic sites.Les tumeurs malignes des tissus mous constituent un groupe tres heterogene compose de plus de 50 entites differentes derivees soit du mesenchyme, soit du neuroectoderme. Les difficultes diagnostiques concernent, d'une part, l'etablissement d'un diagnostic de malignite et, d'autre part, la determination precise du type de sarcome. Le pronostic est en grande partie determine par l'extension locale et a distance de la maladie. L'etude du genotype des cellules tumorales a permis de mettre en evidence des alterations genetiques. Ces alterations, frequemment des translocations chromosomiques, constituent des marqueurs specifiques a certains sous-groupes tumoraux et presentent ainsi un interet diagnostique. Ces marqueurs peuvent etre detectes par la cytogenetique, l'hybridation in situ ou la biologie moleculaire. La specificite de ces alterations pour les cellules tumorales et la possibilite de les mettre en evidence par la technique sensible de reaction en chaine a la polymerase (polymerase chain reaction [PCR]) ont permis de developper des tests visant a rechercher les cellules tumorales au niveau de sites potentiellement metastatiques.