In the previous chapter, all factors affecting the intensity of the reflections were discussed, except the principal one—the structure factor, ∣F hkl ∣^2 , which is the square of the absolute value of the structure amplitude F hkl. This chapter reviews in detail the structure factor parameters, such as the population (or occupation) factors of atoms, atomic displacement (or temperature) parameters, atomic scattering factors, and the phase angles of the reflections. It also considers the effects of symmetry on the structure amplitude, including Friedel’s law and the multiplicity of the reflections. Finally, it discusses systematic absences along with tables used for space group determination.