Microbial symbioses, in which microbes have either positive (mutualistic) or negative (parasitic) impacts on host fitness, are integral to all aspects of biology, from ecology to human health. In many well-studied cases, microbial symbiosis is characterized by a specialized association between a host and a specific microbe that provides it with one or more beneficial functions, such as novel metabolic pathways or defense against pathogens. Even in relatively simple associations, symbiont-derived benefits can be context dependent and influenced by other host-associated or environmental microbes. Furthermore, naturally occurring symbioses are typically complex, in which multiple symbionts exhibit coordinated, competing, or independent influences on host physiology, or in which individual symbionts affect multiple interacting hosts. Here we describe research on the mechanisms and consequences of multipartite symbioses, including consortia in which multiple organisms interact with the host and one another, and on conditional mutualists whose impact on the host depends on additional interacting organisms.