Abstract: Underwater acoustic channel is one of the most complex wireless communication channels.The inherent characteristics,such as space-time-frequency varying,narrow-band,high-noise,strong multipath interference,long transmission delay,and large fluctuation,make the effectiveness and reliability of underwater acoustic communication face enormous challenges.In this paper,the features and development of underwater acoustic communications and networks are introduced,the key techniques in UAC and UACN,including modulation,demodulation and signal de-tection analyzed,and the protocol layer in UACN provided.Finally the 4th generation modem made by Benthos and the US Navy‘s Seaweb Program and the perspective on development and application of UAC and UACN in China is illu-minated.