With the development of cancer biology and treatment, it has become essential to search not only for basic information such as histology but also for biomarkers that represent the characteristics of tumors when deciding on a treatment strategy. Circulating tumor cells(CTCs), circulating tumor DNA(ctDNA), exosomes, and microRNAs are present in the blood or other body fluid and reflect the tumor status in real-time. Since liquid biopsy is less invasive and easier to collect, it is almost ready to be applied not only for diagnosis but also for monitoring the acquisition of resistance to treatment in real-time. The number of CTCs has been shown to be a prognostic factor in itself for breast cancer, and ctDNA with whole-genome sequencing of tissue specimens is developed to monitor recurrence using the individualized ctDNA set for each patient. Furthermore, cancer genome profiling tests using ctDNA have been commercialized and can now be used with an insurance reimbursement. The possibility of early diagnosis using blood microRNA has already been reported, and the sensitivity and specificity are currently being verified in a prospective observational with a screening cohort. The liquid biopsy will be essential for future breast cancer treatment.