Provided herein are compositions and methods comprising microbiome biomarkers of responsiveness/resistance to immunotherapy (e.g., anti-PD1/PD-L1 therapy), and diagnostic, prognostic and therapeutic uses thereof. In particular, the amount, identity, presence, and/or ratio of microflora in the microbiome of a subject is used to determine the responsiveness/resistance of the subject to immunotherapy, and/or the microbiome of a subject is manipulated to enhance the responsiveness of the subject to various immunotherapies and co-therapies.