The gut microbiota plays a crucial role in modulating anti-tumor immunity and the response to immunotherapy, particularly immune checkpoint inhibitors (ICIs). Specific gut bacteria can influence the effectiveness of ICIs, but the underlying mechanisms are not fully understood. Early studies have shown that targeting the gut microbiome through various methods, such as fecal microbiota transplantation (FMT) and probiotics, can enhance the efficacy of ICIs, leading to improved clinical outcomes in patients with various cancers. The gut microbiota's impact on ICI efficacy can be either immune-inhibiting or immune-activating, and specific bacterial species, such as *Lachnospiraceae*, *Ruminococcaceae*, *Faecalibacterium*, and *Akkermansia*, have been linked to better responses to ICI treatments. These bacteria can influence host immune responses by altering the composition of the gut microbiota, modulating the production of metabolites like short-chain fatty acids (SCFAs), and affecting the activity of immune cells. Understanding the mechanisms by which these bacteria influence ICI efficacy can help develop more effective strategies to enhance the success of immunotherapy in cancer patients. Future research should focus on identifying specific functions of bacteria rather than their presence or absence, and establishing standardized protocols to compare and integrate findings from different studies.The gut microbiota plays a crucial role in modulating anti-tumor immunity and the response to immunotherapy, particularly immune checkpoint inhibitors (ICIs). Specific gut bacteria can influence the effectiveness of ICIs, but the underlying mechanisms are not fully understood. Early studies have shown that targeting the gut microbiome through various methods, such as fecal microbiota transplantation (FMT) and probiotics, can enhance the efficacy of ICIs, leading to improved clinical outcomes in patients with various cancers. The gut microbiota's impact on ICI efficacy can be either immune-inhibiting or immune-activating, and specific bacterial species, such as *Lachnospiraceae*, *Ruminococcaceae*, *Faecalibacterium*, and *Akkermansia*, have been linked to better responses to ICI treatments. These bacteria can influence host immune responses by altering the composition of the gut microbiota, modulating the production of metabolites like short-chain fatty acids (SCFAs), and affecting the activity of immune cells. Understanding the mechanisms by which these bacteria influence ICI efficacy can help develop more effective strategies to enhance the success of immunotherapy in cancer patients. Future research should focus on identifying specific functions of bacteria rather than their presence or absence, and establishing standardized protocols to compare and integrate findings from different studies.