Metastatic breast cancer is one of most common and lethal cancers for women in the Western world. Tumor formation and subsequent metastasis are highly complex processes which are not only controlled through mutations of the tumor cell, but are rather made possible by the surrounding extracellular matrix and the immune system. Therefore, the focus of this work lies on the role of tumor-associated macrophages and the extracellular matrix, as well as the question which mechanisms and influences drive the development of these tumor cells. Two major hurdles in the treatment of breast cancer patients are disseminated tumor cells, and the fact that breast cancer almost always forms metastases in the bones, lungs, liver and brain of patients. Especially the bone marrow represents a safe haven for disseminated tumor cells where they can settle in a dormant state and remain undetected for decades, despite therapy. The underlying mechanisms of organ specific metastases have so far only been partially understood. On the one hand, metastatic cells can specifically adapt to their new environment, on the other hand some organs have a certain predisposition as site of cancer metastasis due to their anatomical structures.