The paper by Maurice A. Biot presents a unified treatment of the mechanics of deformation and acoustic propagation in porous media, extending his earlier theory of deformation derived from nonequilibrium thermodynamics. The fluid-solid medium is treated as a complex physical-chemical system with relaxation and viscoelastic properties. Specific relaxation models are discussed, and the general applicability of a correspondence principle is emphasized. The theory of acoustic propagation is extended to include anisotropic media, solid dissipation, and other relaxation effects. The paper also discusses the physical significance of the operators and provides examples to illustrate their flexibility and generality. The stress-strain relations for isotropic and anisotropic media are derived, and the relationship between the effective stress and the porosity is explored. The paper concludes with a discussion of the physical significance of the operators and their application to various types of viscoelastic behavior.The paper by Maurice A. Biot presents a unified treatment of the mechanics of deformation and acoustic propagation in porous media, extending his earlier theory of deformation derived from nonequilibrium thermodynamics. The fluid-solid medium is treated as a complex physical-chemical system with relaxation and viscoelastic properties. Specific relaxation models are discussed, and the general applicability of a correspondence principle is emphasized. The theory of acoustic propagation is extended to include anisotropic media, solid dissipation, and other relaxation effects. The paper also discusses the physical significance of the operators and provides examples to illustrate their flexibility and generality. The stress-strain relations for isotropic and anisotropic media are derived, and the relationship between the effective stress and the porosity is explored. The paper concludes with a discussion of the physical significance of the operators and their application to various types of viscoelastic behavior.