This article is a review of the relevant literature published between April 2019 and March 2020 on the impact of cancer therapy on the gut microbiota and the effects of microbiota modulation on the outcome of cancer treatment. The decrease in richness and diversity of the gut microbiota is associated with a higher rate and degree of side effects in patients undergoing classic systemic chemotherapy. In patients receiving immuno-therapy, higher diversity of the gut microbiota is associated with treatment response, often then resulting in prolonged survival. Studies confirmed a negative impact of antibiotic treatment on the response to immuno-therapy and respective patient outcomes. Part of these effects seems to be mediated by a direct interaction between the gut microbiota and local immune cell populations in the intestinal mucosa, in particular T-cell and macrophage populations. A less diverse gut microbiota was also associated with a higher prevalence of side effects of radiotherapy treatment. Data from animal experiments indicate beneficial effects of antibiotic treatment prior to radiotherapy, which is in contrast to the data from patients receiving immuno-therapy. Studies published within the timeframe addressed in this review suggest that faecal microbiota transplantation can improve treatment-induced diarrhoea. In haematological malignancies, chemotherapy and allogeneic hematopoietic cell transplantation lead to decreased richness and loss of diversity of the gut microbiota. As expected, the lowest microbiota diversity was seen in patients receiving antibiotics. A lower diversity before and after cancer treatment was associated with poor survival. The gut microbiota may serve as a target for manipulating the immune outcome response to different cancer treatment modalities with the aim to improve the efficacy of oncological therapy.