Microbial communities are ubiquitous and associate with every macroorganism examined to date. Recent technological advances have revealed the diversity of microbial communities, including bacteria, fungi, and viruses that associate with macroorganism hosts, and a growing body of work explores the impacts symbiotic, non-pathogenic, microbes can have on host phenotype, interactions, and fitness. In some cases, microbial communities substantially influence phenotypes previously ascribed to macroorganisms. However, symbiotic microbial effects on host phenotype and fitness vary substantially in the strength and direction. The objective of this symposium is to examine the contexts (e.g. ecological, genomic, evolutionary, environmental) that may determine the effects of symbiotic microbial communities on their hosts. In this session, we will bring together researchers working on diverse interactions using complementary techniques including experimental, genomic, metagenomic, and evolutionary to uncover when and which microbial partners influence their hosts’ phenotype.