There is growing evidence that accumulation of senescent cells in tissues may promote cancer in aged organisms. It is also generally believed that the growth of malignant tumors is slower and metastasis occurs less frequently in elderly than in young individuals. Here, experimental data on the effect of aging on the growth of transplanted tumors are discussed. No uniform pattern of behavior of tumor cells in old organisms can be observed. While some tumors grow faster in young compared to old animals, others grow slowly. This difference appears to depend on tumor and host factors. There is evidence that the implantation of metastatic tumor cells depends on humoral host factors, whereas the growth rate of metastasis mainly depends on local (microenvironmental) host factors. Age-associated changes in both humoral and local host factors are critical to the behavior and progression of transplanted tumors in the old host.