Book excerpt: Metastasis is the primary cause of mortality in breast cancer patients but the exact mechanisms governing malignant progression remain unclear. Recent studies have highlighted the role of the tumor microenvironment in promoting metastatic tumor growth. In particular, tumor associated macrophages (TAMs) are known to actively collaborate with tumor cells in mediating several steps of the metastatic cascade. Deciphering how tumor cells rewire regulatory networks in tumor associated macrophages (TAMs) will be critical for the development of effective therapies to combat metastatic disease. CSF1 or M-CSF is a key cytokine implicated in driving the proliferation, survival and differentiation of macrophages. It is well established that CSF1 plays a major role in the recruitment of TAMs to the primary tumor site. However, specific regulatory networks activated by CSF1 in the metastatic milieu are yet to be delineated.