The extension of a cancer is a major prognostic factor which determines the therapeutic strategy. The occurrence of metastatic relapses in patients with initially localized tumours, despite a good local control, gives evidence for the possibility of spreading of occult tumour cells. The recent improvements of immunohistochemistry and molecular biology methods enable to detect tumour cells in various sites such as lymph nodes, bone marrow and blood with a considerably increased sensitivity as compared to conventional approaches. The markers used to detect tumour cells by PCR or RT-PCR can be either "tissue-specific" or "tumour specific". The drawback of the first group of markers is linked to the observation that tissue-specificity is frequently a relative concept leading to a high rate of false positives. Tumour-specific markers include gene fusions observed in various sarcomas, point mutations and presence of viral genomes in tumour cells. They are available and can be easily monitored in only a limited set of cancers. This review focuses on the molecular biology approaches which are used to detect occult tumour cells and on their clinical applications. The large number of studies which have been published in that field show that such a detection can be performed in a variety of target sites. However, results of studies performed on larger series of patients together with a better standardization of technics are necessary before they can be used for individual staging of patients.