Sponges can host abundant and diverse communities of symbiotic microorganisms. In this chapter, we review recent work in the area of sponge–microbe symbioses, focusing on (1) the diversity of these associations, (2) host specificity, (3) modes of symbiont transmission, and (4) the positive and negative impacts of symbionts on their hosts. Over the past 4 years, numerous studies have catalogued the diversity of sponge–microbe symbioses, challenging previous hypotheses of a uniform, vertically transmitted microbial community and supporting a mixed model of symbiont community transmission. We emphasize the need for experimental manipulations of sponge–symbiont interactions coupled with advanced laboratory techniques to determine the identity of metabolically active microbial symbionts, to investigate the physiological processes underlying these interactions, and to elucidate whether symbionts act as mutualists, commensals, or parasites. The amazing diversity of these complex associations continues to offer critical insights into the evolution of symbiosis and the impacts of symbiotic microbes on nutrient cycling and other ecosystem functions.