“Mechanics of Materials” is the study of the relationship between the external loads and internal forces experienced by any structure. The magnitude of the internal force at a point in the structure can be quantified as “stress.” In this course, we introduce stress in tensor format first, followed by how and why the stress tensor is transformed to principal stress and equivalent stress. We also discuss the balance of the internal forces, which leads to local equilibrium, which is the basic governing equation in stress analysis. Understanding local equilibrium gives us a good foundation to study the computational simulation of stress analysis.