Circulating tumor cells (CTCs) can be detected with sensitive immunocytological and molecular methods and can potentially cause distant metastases. Consecutively the prognostic significance of CTC-counts for survival was demonstrated in metastatic prostate cancer (mPC) revealing CTCs as reliable surrogate marker during therapy. Comparatively the prognostic value of a CTC-threshold with < 5 vs. ≥ 5 CTCs was superior to the commonly used PSA-decrement algorithms. In contrast despite evidence of CTCs in localized PC, their clinical value in this stage is currently precarious. Furthermore, CTCs may serve as predictive markers with the ability to predict treatment sensitivity or resistance, since they may represent the heterogeneous molecular signature of primary as well as metastatic cancer lesions. Thus, the isolation of CTCs may serve not only as a prognostic tool but moreover as a liquid biopsy and a window towards personalized treatment. This review discusses the clinical impact of CTCs in the different stages of PC.