This paper presents a model for the behavior of nonlinearly hardening materials under complex loadings, particularly cyclic loadings. The authors, Y. F. Dafalias and E. P. Popov, make observations on the macroscopic behavior of materials subjected to uniaxial random cyclic loadings and generalize these observations to construct a model for complex multiaxial loadings. The model introduces the concept of a bounding surface in the stress space that always encloses the loading surface. A parameter defined by the relative position of the loading and bounding surfaces, along with the plastic work done during the most recent loading, determines the value of the plastic modulus. The paper emphasizes the importance of accurately describing material behavior in the plastic range, especially for complex loading histories, and discusses the limitations of existing models. It also reviews historical models of yield and loading surfaces, highlighting the need for a more comprehensive model that captures the changes in work-hardening plastic moduli.This paper presents a model for the behavior of nonlinearly hardening materials under complex loadings, particularly cyclic loadings. The authors, Y. F. Dafalias and E. P. Popov, make observations on the macroscopic behavior of materials subjected to uniaxial random cyclic loadings and generalize these observations to construct a model for complex multiaxial loadings. The model introduces the concept of a bounding surface in the stress space that always encloses the loading surface. A parameter defined by the relative position of the loading and bounding surfaces, along with the plastic work done during the most recent loading, determines the value of the plastic modulus. The paper emphasizes the importance of accurately describing material behavior in the plastic range, especially for complex loading histories, and discusses the limitations of existing models. It also reviews historical models of yield and loading surfaces, highlighting the need for a more comprehensive model that captures the changes in work-hardening plastic moduli.