This chapter provides the fundamental description of one-dimensional bar members. Based on the three basic equations of continuum mechanics, i.e., the kinematics relationship, the constitutive law (here: one-dimensional Hooke’s law for isotropic materials) and the equilibrium equation, the partial differential equation, which describes the physical problem, is derived. In addition to the classical notation, a formal notation is introduced. This allows to highlight the similarities between the different structural members.