In the past ∼155 years, professional and amateur malacologists have recorded ca. 90 described species of sacoglossan opisthobranchs in ∼25 genera on Japanese shores. In addition, there are at least 20 to 40 undescribed or unrecognized sacoglossans also recorded. The extraordinary species richness has been a source of admiration as well as vexation. Worldwide scientific excitement in this group was largely due to two pivotal discoveries by Japanese researchers: (1) the acquisition and retention of functional chloroplasts by the sacoglossan Elysia atroviridis Baba, 1955 and (2) the existence of extant populations of bivalved sacoglossans (initially Tamanovalva limax Kawaguti and Baba, 1959 and then related taxa). Eight of the nine sacoglossan families recognized by Jensen (1996, 2007) are represented in Japan. All the recognized sacoglossan genera are represented in Japan except: Roburnella Marcus, 1982; Platyhedyle Salvini-Plawen, 1973; Gascoignella Jensen, 1985; Olea Agersborg, 1923; Limapontia Johnston, 1836; and the Australian genera Edenttellina Gatliff and Gabriel, 1911 and Midorigai Burn, 1960. Taxonomic uncertainty has been caused by the absence of vouchers, incomplete and/or questionable descriptions, photographic misidentifications (books and internet), chronically unstable classification, and other scientific challenges; in particular, the small size, cryptic coloration, and patchy distribution of sacoglossans have contributed to limited collections of many species. Since 2000, we have collected, photographed, and preserved unusually large numbers of Japanese sacoglossans, including species traditionally considered rare by malacologists. Although it is premature to produce a comprehensive inventory of the Japanese sacoglossan fauna, we consider it necessary to describe explicitly the strengths and weaknesses of current information. This assessment should assist professional and amateur malacologists with future sacoglossan study, particularly in the areas of biogeography, phylogeny, and ecology.