Demosponges of the genus Halichondria Fleming (1828) are common in coastal marine ecosystems worldwide and have been well-studied over the last decades. As ecologically important filter feeders, Halichondria species represent potentially suitable model organisms to link and fill in existing knowledge gaps in sponge biology, providing important novel insights into the physiology and evolution of the sponge holobiont. Here we review studies on the morphology, taxonomy, geographic distribution, associated fauna, life history, hydrodynamic characteristics, and coordinated behavior of Halichondria species.