With the rapid development of next-generation sequencing, deeper insights are being gained into the molecular evolution that underlies the development and clinical progression of breast cancer. It is apparent that during evolution, breast cancers acquire thousands of mutations including single base pair substitutions, insertions, deletions, copy number aberrations, and structural rearrangements. As a consequence, at the whole genome level, no two cancers are identical and few cancers even share the same complement of “driver” mutations. Indeed, two samples from the same cancer may also exhibit extensive differences due to constant remodeling of the genome over time. In this review, we summarize recent studies that extend our understanding of the genomic basis of cancer progression. Key biological insights include the following: subclonal diversification begins early in cancer evolution, being detectable even in in situ lesions; geographical stratification of subclonal structure is frequent in primary tumors and can include therapeutically targetable alterations; multiple distant metastases typically arise from a common metastatic ancestor following a “metastatic cascade” model; systemic therapy can unmask preexisting resistant subclones or influence further treatment sensitivity and disease progression. We conclude the review by describing novel approaches such as the analysis of circulating DNA and patient-derived xenografts that promise to further our understanding of the genomic changes occurring during cancer evolution and guide treatment decision making.