The textbook "Principles of Hydroacoustics" is devoted to the study of the equations of liquid and solid elastic (isotropic and anisotropic). The textbook provides the main characteristics of the reflectivity of ideal and elastic bodies as the simple forms (sphere, infinite cylinder, prolate and oblate spheroids) and bodies of non-analytical shape (finite cylinder with hemispheres at the ends). Moreover, in the textbook, along with classical methods of diffraction theory, such numerical methods as the finite element method and the boundary element method are used. The textbook also studies the theory of synthesis of hydroacoustic antennas and criteria for acoustic diffraction measurements in a hydroacoustic basin.