The human gut is host to a diverse range of microorganisms that offer protection against colonization by multidrug-resistant bacteria. Antibiotic use, medications, health conditions, and lifestyle factors can alter the composition of the gut microbiota in such a way that results in loss of colonization resistance and increased susceptibility to invading pathogenic antibiotic-resistant bacteria. Therapeutics aiming to restore a diverse and protective microbiome are fast advancing. In this review, we focus on the compositional changes within the gut microbiome that are associated with colonization resistance and discuss their use as potential targets for therapeutics or diagnostics.