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Sponges live in tight associations with associated microbes thriving within their bodies. In this chapter, we discuss these intertwined host-symbiont relationships and how microbes influence multiple features of sponge physiology. We start by describing the vast and specific microbial diversity harbored by sponges, which include archaea, bacteria, eukaryote, and virus taxa; how this diversity is influenced by the mode of symbiont transmission; and the environmental factors that shape these microbial communities. Furthermore, we showcase the links between symbiont density and sponge physiology by comparing distinctive traits (e.g., pumping rates, feeding strategies, nutrient cycling) across the continuous spectrum of high (HMA) and low microbial abundance (LMA) sponge holobionts. Second, we present the functions performed by the 130sponge-associated microbes, focusing on models for metabolic exchanges …