The resident microbial community, harboured by humans in sites such as the skin and gastrointestinal tract, is enormous, representing a candidate environmental factor affecting susceptibility to complex diseases, where both genetic and environmental risk factors are important. The potential of microorganisms to influence the human immune system is considerable, given their ubiquity. The impact of the host–gene–microbe interaction on the maintenance of health and the development of disease has not yet been assessed robustly in chronic inflammatory conditions. PsA represents a model inflammatory disease to explore the role of the microbiome because skin involvement and overlap with IBD implicates both the skin and gastrointestinal tract as sources of microbial triggers for PsA. In parallel with genetic studies, characterization of the host microbiota may benefit our understanding of the microbial contribution to disease pathogenesis—knowledge that may eventually inform the development of novel therapeutics.