Publisher Summary This chapter describes the recent advances in cancer-metastasis research. Clinical realities and experimental approaches to the study of the basic mechanisms of metastasis formation are discussed. Results are reported, which led to the development of theoretical concepts. Such results and concepts are evaluated with their possible impact for the design of new treatment strategies and experimental findings from many diverse research fields are also summarized. The process of metastasis is a dynamic event that can be described as a sequence of interrelated steps. Experimental results indicated that malignant cells that migrate and disseminate from the primary organ to distant sites and there eventually develop into metastases have to survive a series of potentially lethal interactions. Intimate tumor–host interactions are reported to take place all along the metastatic process. They are elucidated at the steps of angiogenesis, invasion, organ interaction, dormancy, tumor rejection, and tumor immune escape. The outcome of such tumor–host interactions seemed to depend on the intrinsic properties of the tumor cells themselves as well as on the responsiveness of the host.