We describe in detail the current trend using monoclonal antibodies to diagnose ovarian cancer either in vitro or in vivo. The approach with such powerful reagents allows to differentiate in vitro tumor histotypes and to detect in peritoneal washings the presence of a few neoplastic cells which characterize the minimal disease. The detection of elevated sera levels of ovarian cancer-associated antigens, such as CA-125 and TAG-72, allows the monitoring, follow-up of these patients and the response to therapy with great accuracy. We focused our attention on the role in vivo of labelled monoclonal antibodies, mainly for diagnostic purposes. Radioimmunoscintigraphy has been found to be more reliable than CT and US to detect foci of disease mainly in patients already treated by surgery, overcoming all the problems usually encountered with these two procedures.