BACKGROUND Breast cancer (BC) is the most common type of cancer in women and the second cause of cancer-related mortality world-wide. The majority of BC-related deaths is due to metastasis. Bone, lung, brain and liver are the primary target sites of BC metastasis. The clinical implications and mechanisms underlying bone metastasis have been reviewed before. Given the fact that BC lung metastasis (BCLM) usually produces symptoms only after the lungs have been vastly occupied with metastatic tumor masses, it is of paramount importance for diagnostic and prognostic, as well as therapeutic purposes to comprehend the molecular and cellular mechanisms underlying BCLM. Here, we review current insights into the organ-specificity of BC metastasis, including the role of cancer stem cells in triggering BC spread, the traveling of tumor cells in the blood stream and their migration across endothelial barriers, their adaptation to the lung microenvironment and the initiation of metastatic colonization within the lung. CONCLUSIONS Detailed understanding of the mechanisms underlying BCLM will shed a new light on the identification of novel molecular targets to impede daunting pulmonary metastases in patients with breast cancer.