Metastasis, the spread of a tumor from its primary site to other parts of the body, continues to be the most significant problem in the field of cancer. Patients who present with metastatic disease or those who develop metastases after successful management of the primary tumor carry a universally grave prognosis. To improve treatment outcomes for these patients a broader understanding of the biology of metastases is necessary. The biological complexity that characterizes metastasis requires complex experimental systems for its study. To a large extent the modeling of this biological complexity is only possible using animal models. The following review will summarize the strengths and weaknesses of available in vivo models of metastasis including transplantable syngeneic mouse and human-mouse xenografts, genetically engineered mice and naturally occurring cancers of companion animals (pet dogs and cats). No single metastasis model is sufficient to answer all questions. As such, the selection of the optimal model(s) for each biological or translational question is necessary.