The blood–brain barrier regulates the movement of various substances between the blood and the brain and therefore has a crucial role in ensuring normal brain function. In both primary brain tumours and brain metastases, the blood–brain barrier is modified to the blood–tumour barrier (BTB), resulting in altered permeability; however, the BTB continues to restrict the penetration of many therapeutic agents into intracranial tumours. Here, Patricia Steeg describes the current knowledge of BTB structure and function and discusses how this knowledge can be translated into improvements in cancer therapy and patient outcomes.