Reproduction in coralline algae is mostly by the formation of tetraspores, spermatia, carpogonia, and carpospores within conceptacles. This chapter describes the formation of conceptacles and the development of their contents. Numerous studies have been made of conceptacles in coralline algae, and they have provided a great deal of information that is useful in understanding phylogeny and classification as well as developmental processes. Although little data exist, coralline algae are obviously readily capable of vegetative reproduction. Pieces broken from nonarticulated coralline algae continue to grow, if conditions are appropriate. Reproductive cells reach maturity within conceptacles in all extant coralline algae except the specialized endophyte Schmitziella. Conceptacle development gives good clues to the evolutionary routes followed by various Corallinaceae. In nonarticulated coralline algae, conceptacles are located on the surface of crusts or on protuberances. For many years post-fertilization development in coralline algae has been shown to involve the formation of a coenocytic fusion cell from which the gonimoblast filaments arise.