Owing to recent technical development of comprehensive genome-wide analysis such as next generation sequencing, deep biological insights of breast cancer have been revealed. Information of genomic mutations and rearrangements in patients’ tumors is indispensable to understand the mechanism in carcinogenesis, progression, metastasis, and resistance to systemic treatment of breast cancer. To date, comprehensive genomic analyses illustrate not only base substitution patterns and lists of driver mutations and key rearrangements, but also a manner of tumor evolution. Breast cancer genome is dynamically changing and evolving during cancer development course from non-invasive disease via invasive primary tumor to metastatic tumor, and during treatment exposure. The accumulation pattern of base substitution and genomic rearrangement looks gradual and punctuated, respectively, in analogy with contrasting theories for evolution manner of species, Darwin’s phyletic gradualism, and Eldredge and Gould’s “punctuated equilibrium”. Liquid biopsy is a non-invasive method to detect the genomic evolution of breast cancer. Genomic mutation patterns in circulating tumor cells and circulating cell-free tumor DNA represent those of tumors existing in patient body. Liquid biopsy methods are now under development for future application to clinical practice of cancer treatment. In this article, latest knowledge regarding breast cancer genome, especially in terms of ‘tumor evolution’, is summarized.