Most of the soil mechanics models developed for agricultural applications have been derived from elastoplastic constitutive models. At the same time there has been the formal development of the science of "unsaturated soil mechanics" (Fredlund and Rahardjo, 1993a). The first part of this chapter will be devoted to a review of stress and strain variables and their utilization in theoretical models of soil as an elastoplastic material. Applications of these models in the agricultural engineering literature have been presented in Volume 1 of this Monograph in the section "Soil Mass Dynamic Load-Deformation Properties Definable within a System of Mechanics (Stress-Strain)" (Chancellor, 1994: 149-238). Theoretical aspects necessary for understanding the concepts discussed in this chapter will be presented here. This part will be followed by a presentation of critical-state soil mechanics. The theories of critical-state soil mechanics were developed from the application of plasticity theory to observed soil behavior. In the first instance, the additional complexities introduced by considering the unsaturated, multiphase, nature of agricultural soils will not be considered (these will be discussed in some depth in the last part of this chapter). The intention is to explain some of the more difficult concepts of the critical-state framework in such a manner that they can be understood conceptually, even if the reader is not greatly interested in the mathematical details of the theory.