Just as waves occur on the surface of the ocean, they occur in its interior. Wherever the density is increasing with depth there may be "internal gravity waves" which result in up-and-down motion of density surfaces and are important in ocean mixing, bottom currents and acoustic propagation. Acoustic waves can propagate throughout the ocean and are the main technique for remotely sensing the ocean interior. This course examines the dynamics, generation and propagation of internal gravity waves and their impact on the ocean environment. We also examine the ways in which acoustic remote sensing is used to study the ocean interior and bottom.