The concept of stress and strain is key to the understanding of deformation. When a force is applied to a continuum medium, stress is developed inside it. Stress is the force per unit area acting on a given plane along a certain direction. For a given applied force, the stress developed in a material depends on the orientation of the plane considered. Stress can be decomposed into hydrostatic stress (pressure) and deviatoric stress. Plastic deformation (in non-porous materials) occurs due to deviatoric stress. Deformation is characterized by the deformation gradient tensor, which can be decomposed into rigid body rotation and strain. Deformation such as simple shear involves both strain and rigid body rotation and hence is referred to as rotational deformation whereas pure shear or tri-axial compression involves only strain and has no rigid body rotation and hence is referred to as irrotational deformation. In rotational deformation, the principal axes of strain rotate with respect to those of stress whereas they remain parallel in irrotational deformation. Strain can be decomposed into dilatational (volumetric) strain and shear strain. Plastic deformation (in a non-porous material) causes shear strain and not dilatational strain. Both stress and strain are second-rank tensors, and can be characterized by the orientation of the principal axes and the magnitude of the principal stress and strain and both have three invariants that do not depend on the coordinate system chosen.