Chapter 3 is one of the most important chapters in the book as it discusses the axial–flexural response and design of cross-sections of different shapes and made up of different materials, in a unified and integrated manner. This treatment of cross-section behavior is unique and not covered in most of the books on this subject. A set of general equation as well as computation procedure is presented to determine the capacity interaction surface of generalized cross-section made up of any number and configuration of materials. The significance of capacity ratio, biaxial bending, P–M curves, M–M curves, and practical applications of this approach are discussed with extensive illustrations and graphics obtained from various software, which are based on the formulation presented in this chapter. Several solved examples are presented, together with flowcharts to demonstrate the code-based procedures for design of beam-column sections. ACI, BS, and Euro code provision are also discussed.