Circulating tumour cells (CTCs) have been considered for a long time in reflecting the aggressiveness of tumours. As a result, many attempts have been made to develop assays that reliably detect and enumerate CTCs, but only recently have such assays been available. The first clinical results obtained with such assays strongly suggest that in some tumour types, CTC detection and enumeration can be used to estimate prognosis and may serve as an early marker to assess anti-tumour activity of a treatment. Furthermore, through technical advances, CTCs can be characterised for several features, which may shortly yield better prognostic and predictive classification systems and may also provide improved insight into biological processes including dissemination, drug resistance and treatment-induced cell death. This review addresses CTCs, and in particular, technical issues concerning their detection, clinical results obtained so far, and future perspectives.