Brain metastases are a major cause of death in breast cancer patients. A key event in the metastatic
progression of breast cancer in the brain is the migration of cancer cells across the blood-brain barrier (BBB). The
BBB is a natural barrier with specialized functions that protect the brain from harmful substances, including antitumor
drugs. Extracellular vesicles (EVs) sequestered by cells are mediators of cell-cell communication. EVs
carry cellular components, including microRNAs that affect the cellular processes of target cells. Here, we summarize
the knowledge about microRNAs known to play a significant role in breast cancer and/or in the BBB
function. In addition, we describe previously established in vitro BBB models, which are a useful tool for studying
molecular mechanisms involved in the formation of brain metastases.