Nearly 20% of breast cancer patients with non-localized disease are eventually diagnosed with brain lesions, making breast cancer a main source of metastatic brain disease in women. The process of metastasis is highly specific, though the molecular basis for breast cancer metastasis to the brain remains unclear. A novel proteomic signature associated with breast cancer brain metastasis showed an association of the expression of neurotrophin-3 (NT-3), a growth factor, with brain-targeting metastatic breast cancer cell lines. To evaluate the importance of NT-3 in the ability of brain-targeting breast cancer cell lines to grow in the brain, I knocked down NT-3 expression in brain-targeting metastatic breast cancer cells and overexpressed NT-3 in non brain-targeting metastatic breast cancer cells. The ability of the cells to grow in the brain was lost in cells lacking NT-3 expressing and the ability was gained in cells overexpressing NT-3. These findings show that NT-3 expression is necessary and sufficient to promote growth in the brain. Currently, breast cancer surface receptors define the prognosis of breast cancer metastasis to the brain. Here, I show expression levels of human epidermal growth factor receptor (HER-2), a marker associated with breast cancer brain metastasis, correlate with levels of NT-3. Treatment of brain-targeting metastatic breast cancer cells with exogenous NT-3 causes activation of HER-2. This shows crosstalk between NT-3 and HER-2, suggesting a mechanism by which NT-3 can promote growth of brain metastatic breast cancer cells. Further findings elucidate a possible role between NT-3 expression by brain metastatic breast cancer cells and the microenvironment. Clinically, breast cancer brain metastases have been shown to recruit microglia, an immune cell of the brain. I show here that microglia activation is lower in the presence of tumors expressing NT-3, while breast cancer with low levels of NT-3 have high levels of activation. I propose that NT-3 can promote growth of metastatic breast cancer cells in the brain through HER-2 activation and via an interaction with the brain microenvironment.