Analyzing the different steps of malignant growth (primary tumor, metastasizing tumor cells, secondary tumor), one recognizes an intense interaction between normal and malignant cells. Tumor cells not only induce activities of normal cells, which normally are rarely activated, but also they exploit properties of normal cells for their own purposes. The major mechanisms and processes of this "parasitism" are described in more detail and the results are discussed. Tumors cannot grow beyond a certain size without a supply of blood and lymph vessels by the host (angiogenesis). Metastasizing tumor cells cannot leave the vessel (extravasate) in which they are transported without the cooperation of the respective endothelial cells of the host. An appropriate environment formed by the host tissues is essential for the settlement of tumor cells at secondary sites. Historically, these are a few examples that show intense cooperation between host and tumor. More are given in the present contribution.