A material or structural component does not exist in isolation. It is subjected to the action of surrounding objects, known as the load. How is a load represented mathematically? How is the load or force represented inside the body of a material when it acts on the material? What is the effect of this force? The effect mentioned here is the effect inside the body, that is, the deformation of the object, not the movement of the object as a whole. Our concern here is with a deformable object, not a rigid object. This chapter is intended to illustrate, by using tensors as a mathematical tool, the fundamental concepts and physics ideas of stress, displacement, deformation, strain, and the stressstrain constitutive relationship under the assumption of a continuous medium.