The intestinal microbiota is an important part of the human body and its composition can affect the effectiveness of immunotherapy. In the last few years, the modulation of intestinal microbiota in order to improve the effectiveness of immunotherapy has become a current topic in the scientific community, but there is a lack of research in this area. In this thesis, the goal was to analyze the current literature related to the modulation of intestinal microbiota and the effectiveness of immunotherapy in the treatment of cancer. The effects of antibiotics, probiotics, diet and faecal microbial transplantation were analyzed separately. From eight current studies that studied the influence of antibiotics on the effectiveness of immunotherapy, it was concluded that the use of antibiotics, especially broad-spectrum and/or use of antibiotics in larger quantities, causes dysbiosis of the intestinal microbiota, which can reduce the effectiveness of immunotherapy. Using five current studies on probiotics and immunotherapy, it can be seen that dysbiosis could be repaired by probiotics and thus affect the improvement of immunotherapy, but the use of commercial probiotics without evidence of intestinal dysbiosis has not yet been sufficiently tested to confirm its safety for cancer patients who are treated with immunotherapy. A diet consisting of a sufficient amount of fiber is recommended by four current studies on the impact of a diet with a higher amount of fiber and the effectiveness of immunotherapy. According to two studies, a diet with a higher salt content positively correlates with the success of immunotherapy. According to the only three human studies, fecal transplantation is a safe and realistic adjuvant option for the treatment of cancer patients with immunotherapy, but it is necessary to standardize treatment protocols and establish more clinical evidence of the success of this type of treatment. Modulating the composition of the microbiota can indeed change the effectiveness of immunotherapy, but in the future, more human studies should be organized to precisely determine the exact types and procedures of microbiota modulation and investigate them in more detail.