Metastatic spread of cancer cells is the main cause of death of breast cancer patients. A better understanding of the molecular mechanism of cancer metastasis is essential for the development of novel and effective therapies. The biological complexity of the metastasis process requires the combination of multiple experimental systems to model distinct steps of cancer metastasis. Several animal models have been generated to mimic the process of breast cancer metastasis, with unique advantages and drawbacks of each model. In this chapter, we describe transplantable xenograft and allograft methods to introduce human or mouse breast tumor cells into mice in order to generate spontaneous and experimental metastasis.