This chapter mainly deals with the basic concepts of constructing cross sections of various polyhedra, such as the trace method and the projection method. The reader will learn about methods of finding the area of a cross section, as well as important theorems to determine such areas using the projected area of a cross section. This chapter explains how to calculate the ratio of volumes of figures in which a cutting plane divides a polyhedron. It demonstrates how some useful auxiliary constructions can help in solving Olympiad-type problems. The reader will see applications of stereometry to construct shadows cast by an object under various light sources, such as a floor or ceiling lamp or shadows cast by the sun’s rays. This chapter is certainly the best for mastering the method of construction and developing spatial imagination.