The biologic mechanisms that individual cancer cells use to metastasize to the brain have been revealed by recent research. Metastasis, the process that allows some cancer cells to break off from their tumor of origin and take root in a different tissue, is the most common reason people die from cancer. Metastatic brain tumors are ten times more common than primary brain cancers. Yet most tumor cells die before they can take root in the brain, which is better protected than most organs against colonization by circulating tumor cells. To seed in the brain, a cancer cell must dislodge from its tumor of origin, enter the bloodstream, and cross densely packed blood vessels called the blood-brain barrier. Until now, little research has been done into how metastatic brain tumors develop.