Structural behavior of reinforced concrete is quantified through relationships between generalized forces and corresponding displacements called constitutive laws. Within the present chapter three levels of constitutive laws are defined: material, section, and member. For each level monotonic as well as hysteretic loading are considered. Basic concepts which allow understanding and quantifying the reinforced concrete behavior peculiarities like: moment curvature relationship, plastic length, plastic hinge, spread plasticity, sectional and member ductility, energy dissipation, stable and unstable hysteretic behavior, are thoroughly examined. Up-to-date models describing the structural response of different types of R/C members, including short and long beams and columns, are presented. Numerical examples illustrating the theoretical concepts are included.