Current knowledge about sexual reproduction in Noctiluca scintillans is reviewed in this paper. Noctiluca scintillans is one of the most prominent "red tide" organisms found in all Chinese coastal waters. There are two forms, red Noctiluca and green Noctiluca. The former is heterotrophic and broadly distributed in temperate and sub-tropical coastal waters. The latter contains a photosynthetic symbiont and can survive without an external food supply. It is mainly limited to the tropical west Pacific, parts of the Arabian Sea, the Gulf of Oman, and the Red Sea. The complete life cycle of N. scintillans consists of binary fission and sexual reproduction. A small fraction of trophonts transform into gametogenic cells, which undergo two successive nuclear divisions. The products of nuclear division migrate to the cell surface with a small amount of cytoplasm and further synchronously divide six to eight times. A large number of gametes, which possess some noctilucid-like characters, such as a transverse groove, a longitudinal groove, and two flagella, are released from the mother cell ghost. The zygote, which results from the fusion of two isogametes, gives rise to miniscule trophonts, which undergo change in shape and reduction in flagellum number and acquire a tentacle and crust. Gamete fusion in N. scintillans is not followed by formation of a non-motile resting cyst or by meiosis. To date, there is some controversy regarding the process of sexual reproduction, i.e., gametogenesis and the development from zygote to trophont. There is a strict program for the formation of gametogenic cells, which starts to operate in every cell after a defined number of binary fissions. However, the presence of gametes can reset this program and keep trophonts in binary fission instead of proceeding to gametogenesis. Sexual reproduction is common when populations are increasing but rare during population decline both in the field and in the culture. Thus, sexual reproduction is considered to contribute to the population growth of N. scintillans. Gametes were found all year round in coastal waters of Sagami Bay, and its peak abundance was observed to occur just before or at the same times as the peak abundance of the trophonts. It is suggested that sexual reproduction enhances the formation of N. scintillans "red tides" as intensive multiplication may occur under high-density conditions. The primitiveness of the noctilucid flagellum can be regarded as an ancestral character of dinoflagellates. If the gametes are likely to reflect the primary attributes of trophonts, it could be concluded that N. scintillans is an ancestral form of dinoflagellates. The methodology of investigating this sexual reproduction, including collection and cultivation, observation by light microscopy, quantification of gametes, etc., is briefly introduced in this article. Studies on physiology, population dynamics, and bloom formation of N. scintillans in Chinese coastal waters are summarized, and future research topics are briefly discussed.