Freshwater mussels (Mollusca: Bivalvia: Unionoida) are a species-rich group of parasitic bivalves comprising approximately 843 nominal species in six families, including 300 species of Unionidae and five of Margaritiferidae in North America. Unionid shells have been studied extensively for the purposes of taxonomy, but less information exists about the cellular anatomy of their “soft tissues” (mantle cavity tissues and visceral tissues). No systematic histological atlas of any unionid has been published in the peer-reviewed literature, and this lack of information hinders basic and applied research topics involving freshwater mussels. Herein, we describe the tissue and cell anatomy of a representative species from each of three lineages (tribes) of Unionidae sensu Graf & Cummings (2006) ranging in North America: Villosa nebulosa (Ambleminae: Lampsilini), Fusconaia cerina (Ambleminae: Pleurobemini) and Strophitus connasaugaensis (Unioninae: Anodontini). Based on necropsy observations and light microscopy of serial histological sections, for each species we describe and compare mantle cavity tissues (i.e., tissue enclosed by mantle: mantle, adductor muscle, pedal protractor, pedal retractor, gill, foot, labial palp) and visceral organs (i.e., internal organs: esophagus, digestive diverticulum, stomach, crystalline style sac, intestine, heart, nephridium, cerebral ganglia, pedal ganglia, visceral ganglia, ovaries and testes). We also present a synoptical review of pertinent literature on histological anatomy of unionids. The present study (i) represents the first histological atlas for freshwater mussels, (ii) comprises a baseline for monitoring mussel health in aquatic ecosystems, and (iii) could assist future workers studying freshwater mussel physiology, life history, toxicology, pathology, systematics and ecology.