Colorectal cancer (CRC) is a heterogeneous disease of the intestinal epithelium that is characterized by the accumulation of mutations and a dysregulated immune response. Up to 90% of disease risk is thought to be due to environmental factors such as diet, which is consistent with a growing body of literature that describes an 'oncogenic' CRC-associated microbiota. Whether this dysbiosis contributes to disease or merely represents a bystander effect remains unclear. To prove causation, it will be necessary to decipher which specific taxa or metabolites drive CRC biology and to fully characterize the underlying mechanisms. Here we discuss the host-microbiota interactions in CRC that have been reported so far, with particular focus on mechanisms that are linked to intestinal barrier disruption, genotoxicity and deleterious inflammation. We further comment on unknowns and on the outstanding challenges in the field, and how cutting-edge technological advances might help to overcome these. More detailed mechanistic insights into the complex CRC-associated microbiota would potentially reveal avenues that can be exploited for clinical benefit. This Review describes the interplay between host genetics, host immunity and the gut microbiome in the modulation of colorectal cancer, and discusses the role of specific bacterial species and metabolites alongside technological advances that will facilitate more in-depth investigation of the microbiome in disease.