This chapter covers the current knowledge on interkingdom communication in marine sponge holobionts. Sponges (Porifera) are known to contain dense and diverse microbial symbiotic communities located extracellularly in the sponge tissue. Despite their early evolutionary origin, sponges already possess a diverse array of immune genes involved in host–microbe interactions. Besides sponge–bacteria interactions, quorum sensing and quorum quenching molecules have been identified in sponge-derived bacterial isolates and genomes that mediate bacterial signaling. Thirdly, a tripartite interaction of sponge cells, bacteria, and bacteriophages has recently been discovered that provides a functional understanding of phage-mediated immune evasion by bacteria on the cellular level. This review discusses the various types of interactions (host–microbe, microbe–microbe, phage–microbe–eukaryote) in sponge holobionts, which are considered to be just the iceberg of a plethora of possible interactions awaiting discovery in sponge–microbe interactions.