Cancer heterogeneity, long recognized as an important clinical determinant of patient outcomes, was poorly understood at a molecular level. Genomic studies have significantly improved our understanding of heterogeneity, and have pointed to ways in which heterogeneity might be understood and defeated for therapeutic effect. Recent studies have evaluated intratumoral heterogeneity within the primary tumor, as well as heterogeneity observed between primary and metastasis. The existence of clonal heterogeneity in the primary and metastasis also affects response to therapy, since the Darwinian pressures of systemic therapy result in clonal selection for initially rare variants. Novel technologies (such as measurements of circulating tumor cells and circulating tumor DNA) may allow physicians to monitor the emergence of clonal subtypes and intervene at an early point to improve patient prognosis.