The enumeration of circulating tumor cells has long been regarded as an 
attractive diagnostic tool, as circulating tumor cells are thought to reflect 
aggressiveness of the tumor and may assist in therapeutic decisions in patients 
with solid malignancies. However, implementation of this assay into clinical 
routine has been cumbersome, as a validated test was not available until 
recently. Circulating tumor cells are rare events which can be detected 
specifically only by using a combination of surface and intracellular markers, 
and only recently a number of technical advances have made their reliable 
detection possible. Most of these new techniques rely on a combination of an 
enrichment and a detection step. This review addresses the assays that have been 
described so far in the literature, including the enrichment and detection steps 
and the markers used in these assays. We have focused on breast cancer as most 
clinical studies on CTC detection so far have been done in these patients.