In this chapter, cancer research using immunodeficient mice, with emphasis on metastasis field studies, is described. The definition of "humanized mice" used for biomedical research is given in the chapter by Nomura et al. in this volume. Briefly, a humanized mouse possesses human cells or tissues and shows, in part, an identical biological function to human beings. However, humanized mice described in this chapter may differ slightly from this definition. In research on cancer metastasis, the in vivo dynamic state of human cancer cells after transplantation into NOG mice is partially identical to that in human beings. This chapter also describes the superiority of NOG mice over conventional immunodeficient NOD/Shi-scid mice in cancer research.