I Principles of thermohydrodynamic description of internal gravity waves in the ocean.- 1. Brief Information About Oceanic Thermohydrodynamics.- 2. Equations of the Theory of Internal Waves.- II Linear theory of internal waves.- 3. Linear Theory of Propagation of Internal Waves in the Undisturbed Horizontally Homogeneous Ocean.- 4. Shear Flow Influence on Internal Waves Propagation.- 5. Propagation of Internal Waves in Horizontally Inhomogeneous Ocean.- 6. Basic Sources of Internal Waves Generation in the Ocean.- III Nonlinear theory of internal waves.- 7. Hamiltonian Formalism for the Description of Oceanic Internal Waves.- 8. Long Weakly Nonlinear Internal Waves.- 9. Propagation of Weakly Nonlinear Internal Wave Packets in the Ocean.- 10. Resonant Interactions of Wave Triads and Kinetic Equation for Oceanic Internal Waves’ Spectrum.- IV Some information on internal wave observations in the ocean.- 11. On Statistical Description of Natural Internal Waves Data.- 12. Basic Experimental Facts of Internal Waves’ Behaviour in the Ocean.- References.- Author Index.- Topic Index.