Over the last decade, immune checkpoint inhibition (ICI) dramatically impacted the treatment of cancer, achieving unprecedented improvements in overall and progression-free survival across a range of advanced and metastatic tumor types. However, despite the impressive efficacy of this treatment modality, the clinical benefit and potential for a long-term response to ICI is restricted to only a subgroup of patients. In this respect, reliable biomarkers that can predict clinical responses to immunotherapy are urgently needed. In recent years, evidence is emerging suggesting that the gut microbiome can modify the efficacy and toxicity of ICI. This article will briefly review the complex interface between the microbiome and the immune system and its effects on ICI, and explore the potential of manipulating the gut microbiome in an attempt to improve the efficacy and safety of ICI.