The authors review the principles of systemic therapy in breast cancer. They analyze the degree of treatment individualization in our current approach. New technologies allow the detection of genomic alterations in cancer cells. Unfortunately, we do not know yet how to best use this knowledge for routine patient care. Most genomic alterations are rare events complicating further drug development. Temporal and spatial heterogeneity in tumors also has to be taken into account. An intense international collaboration is ongoing to try and demonstrate that precision medicine will really improve treatment outcome.