This chapter provides the basis for the study and understanding of the structural cross-sections. The anatomy and hierarchy of the cross-section shapes, types, and the materials they are made up is extensively covered. Several methods for defining the cross-sections, including parametric and general procedures, are discussed. The main focus of the chapter is to provide closed-form solutions for computing the geometric properties of various section shapes. It also discusses general polygon and mesh-based procedures for solution of composite and complex sections. In addition to the formulae to compute the properties, a discussion on the significance of various properties helps us to improve the cross-sectional behavior of computer-based methods, and several solved examples are included followed by problems for readers to try and explore. The chapter makes use of extensive illustrations and diagrams.