In temperate neritic ecosystem, the phytoplankton community is mainly composed of two dominant algal taxa, diatoms and dinoflagellates, and they play an important role as a primary producer. Of these, dinoflagellates are known to cause red tide outbreaks and even to produce toxin. Recently, red tide events have frequently occurred in several embayments of the Korean coast and have brought serious damage to inshore fisheries. Thus, the red tide research activities including the taxonomy as well as distribution of toxic dinoflagellates have received ever increasing attention in Korean waters. A taxonomic survey of the dinoflagellate family Ceratiaceae was conducted on 17 locations off the coast of Korea. This monograph is the result of the microscopic analyses. A total of seventeen species and three varieties have been identified and described, of which Ceratium arcticum, C. longirostrum, C. praelongum, C. tripos var. ponticum are new records for Korea.