Advances in whole genome amplification and next-generation sequencing methods have enabled genomic data analysis of single cells. Meanwhile, these techniques are now beginning to be utilized to detect genomic lesions in individual cancer cells. Previous approaches have been unable to resolve genomic differences in complex mixtures of cells, such as heterogeneous tumors, despite the significance of characterizing such tumors for cancer treatment. Sequencing of single cells(single-cell seq) is likely to improve several aspects of medicine, including the early detection of rare tumor cells, monitoring of circulating tumor cells (CTCs), measuring intratumor heterogeneity, and guiding chemotherapy. There are also lots of challenges and technical aspects of single-cell sequencing,and we also need to know how this information can be used to diagnose and treat cancer patients.The clinical value of single-cell genomic methods will be in monitoring CTCs, profiling rare cancer cells in clinical samples, and detecting rare clones that may be resistant to chemotherapy. These applications are likely to improve all the three major themes of oncology: detection, progression, and prediction of therapeutic efficacy.