The heterogeneous tumor stroma can support therapy resistance at multiple levels. Critical components of the stroma are here the fibroblasts and in particularly CAFs. The aim of the present study was to investigate if and how fibroblasts affect the radiation response of tumor cells. Therefore, a general approach was used where different tumor cell entities in combination with different fibroblasts were analyzed for their behavior upon radiation. The influence of the cells on each other (tumor promoting versus suppressing) were also analyzed in vitro and in vivo. First, in vitro experiments aimed to determine the influence of fibroblasts on the tumor cell survival, cell death, and proliferation after irradiation using indirect (transwell) and direct cell co-culture (3D model) of different fibroblasts/cancer cells combinations. In indirect co-culture, paracrine signals from embryonic NIH-3T3 fibroblasts promoted MPR31.4 prostate cancer cell proliferation and clonogenic survival whereas it impaired the proliferation and favorited cell death of B16F10 melanoma cancer cells after radiation. Moreover, paracrine signals from L929 skin fibroblasts induced higher levels of apoptosis in irradiated MPR31.4 cells, although these cells promoted proliferation and clonogenic survival of irradiated Py8119 cells. Co-culture of cancer cells and fibroblasts resulted in different effects depending on the respective cell sets used. NIH-3T3 co-cultured with MPR31.4 and L929 with Py8119 revealed a tumor-promoting effect when L929 co-cultured with MPR31.4 and NIH-3T3 with B16F10 revealed a tumor-suppressing effect. The results could be confirmed in vivo. Co-implantation of MPR31.4 cells and NIH-3T3 fibroblasts or Py8119 cells and L929 fibroblasts led to increased tumor growth and reduced radiation-induced tumor growth delay when compared to tumors without fibroblasts. In contrast, co-implantation of MPR31.4 cells with L929 fibroblasts had neither effect on tumor growth nor radiation-induced tumor growth delay. NIH-3T3 induced MPR31.4 prostates radio-resistance and L929 induce Py8119 breast radio-resistance when L929 had no effect on MPR31.4 radiation response. The impact of fibroblasts on cancer cell behavior and radiation response largely depends on the combinations of the respective cell types used as they either exert a pro-tumorigenic effect, an anti-tumorigenic effect or no effect. The plasticity of CAFs ranging from tumor stimulation/radio-resistance to tumor inhibition/radio-sensitivity, which is largely influenced by paracrine communications with the cancer cells, allows for such a broad spectrum of activities by the same fibroblast-type.