is MADE of internal-interface or boundary-waves generated by a density current flowing down a bottom with a small slope. The mathematical model and the experimental model each consist of two homogeneous layers of fluid of different densities and viscosities. A summary of previous studies related to internal waves is presented, including theoretical and experimental work as well as in-situ observations.The problem is investigated theoretically for three main cases: Case A.Two superposed parallel layers of fluid-the undisturbed flow is parallel to the interface, and the disturbed flow is associated with long internal waves. Case Bl. Two superposed layers of fluid-long internal waves generated by a steady, uniform density current in the lower fluid. Case B2. This case is similar to Case Bl, but for short internal waves.Each case is resolved into an undisturbed motion plus a perturbation. Equations are developed for the vertical distribution of velocity and interfacial velocity in the undisturbed laminar flow. The basic tool of the analysis is the small disturbance theory in connection with the non-dimensional, linearized Navier-Stokes equations in a two-dimensional, unsteady flow of an incompressible viscous fluid. An appropriate stream function for each fluid is chosen to satisfy the continuity equations. Wave velocities are obtained by introducing the stream function in the equations of motion combined with the applicable boundary conditions at the free surface, interface, and bottom.The theoretically predicted behavior of internal waves as well as the magnitude of their velocities, the vertical distribution of velocity of the two fluids, and the interfacial velocity are qualitatively tested by experimenal work in a tank tilted one degree and five degrees. Wave frequencies and amplitudes are observed and measured.During the experiments it was possible to maintain a steady, uniform density current in the lower fluid. It was observed that accumulated dust arrests practically any possible motion on the free surface.A brief theoretical analysis of internal waves generated in a water-mass, where the density changes with respect to depth, is included.