Abstract: Internal waves, i.e. waves moving within the ocean’s interior, have different physical features from ocean surface waves, and can cause great fluctuation of the temperature profile in the water. This temperature fluctuation has high directivity and varies markedly with time, strongly affecting underwater acoustic propagation. Internal waves give rise to some very interesting acoustic waveguide effects (including 2- and 3-dimensional effects), which greatly influence the detection and localization of underwater targets, as well as acoustic communications. Research on these effects of internal waves involves two disciplines: ocean dynamics and underwater acoustics. In this paper,we first review the effect of internal waves on sound propagation. Then some discussion about the typical acoustic waveguide effect is given. Finally, future prospects of this field are summarized.