The bony fish Danio rerio (zebrafish) has become one of the important vertebrate model organisms in biomedical cancer research and is used, among other things, for the development of anticancer drugs using xenotransplantation approaches. The ex utero development of zebrafish, optically transparent tissues in the first month of growth, and the immature adaptive immune system during this period greatly facilitate the manipulation of embryos. For highly aggressive cancers where patient survival may be expected to be only a few months, a zebrafish xenograft assay may be the only appropriate method as it requires only four to seven days. Thousands of embryos can be implanted with biopsy tissue from a patient to produce zebrafish xenografts and to use them to screen a large number of drugs and compounds automatically to develop an effective treatment regimen for a specific patient. This review examines the advantages and disadvantages of the zebrafish model in oncology research. The main focus is on the use of zebrafish xenografts to study metastasis and to create avatars in personalized medicine.