Metastatic relapse in children with solid tumors is mainly caused by systemic pretreatment dissemination of occult tumor cells. Therefore the initial detection of undetected metastases could have a clinical impact on the prognosis (i.e. new initial staging) and therapy for children with cancer. At later stage it is useful to determine the presence and change in the number of residual malignant cells in order to adjust and/or select adjuvant therapies and techniques (i.e. autologous bone marrow transplantation, leukapheresis.). Over the past decade, sensitive immunocytochemical and molecular assays have been developed which permit the identification of disseminated cancer cell. Actually tumor cell contamination can be detected in bone marrow or in peripheral blood of children with following cancers: neuroblastoma, Ewing tumor, alveolar rhabdomyosarcoma, PNETs. In this review, focus is on the recent technical achievements in the detection of occult cancer cells in bone marrow and in blood and a discussion of their usefulness for clinical trials.