Abstract: Due to severe attenuation, the transmission distance of electromagnetic waves is significantly limited in water. Hence, underwater acoustic communication has become the dominant approach for achieving wireless communication in the ocean. Underwater acoustic communication is very important for both military and civilian usage, and with the speeding up of marine development,has seen an ever increasing demand. An overview of the challenges in underwater acoustic communications is presented in this paper. We introduce some basic concepts, emphasizing the properties of underwater acoustic channels and current challenges. Then, several typical techniques are presented. To conclude, some future developments are described.