"Liquid biopsy" has been introduced for the analysis of circulating tumor cells (CTCs) in the blood of cancer patients, and this new diagnostic approach has received enormous attention. CTCs are derived from primary tumors and metastatic lesions and, therefore, harbor important information on the molecular characteristics relevant to tumor progression and cancer therapy. Analyses of CTCs have paved new diagnostic avenues with obvious clinical implications for personalized medicine. Key areas of clinical applications of CTCs include detection of cancer, prediction of prognosis in patients with curable disease, monitoring systemic therapies, and stratification of patients based on the detection of therapeutic targets or resistance mechanisms. The present Review will focus on the current knowledge derived from single-cell analysis of CTCs at the DNA, RNA, and protein level and emphasize the unique opportunities for performing functional studies that aim to identify the metastasis-initiator cells in cancer patients and obtain predictive information for steering cancer therapies.