In the last 5 years, the intestinal microbiome has been described as a major driver and modulator of intestinal but also systemic immunoregulation. Based on the literature and our own experience, we summarize the current knowledge of the role of the microbiome in pathophysiology of solid cancers and in various types of immunotherapy for the treatment of hematological and solid malignancies: Immunological side effects of allogeneic stem cell transplantation but also of treatment of solid cancers with checkpoint inhibitors are strongly controlled by intestinal microbiota. Furthermore, recent evidence indicates the contribution of intact microbiota and microbiota-dependent immunoregulation to efficacy and control of tumor growth by checkpoint inhibitors
and cellular immunotherapy. Further research addressing microbiota strain-specific mechanisms and metabolites is needed to understand the role of tumor-specific microbiota and to utilize these interactions for optimization of immunotherapy of cancer.