In order to model experimental extended X-ray absorption fine-structure (EXAFS) data to extract quantitative structural parameters N, R and σ² for the neighboring atoms, accurate functions for the photoelectron scattering phase shifts and amplitudes are required. These depend strongly on the photoelectron wavenumber k and on the species (Z) of the absorbing and scattering atom, and to a somewhat lesser degree on the distance R to the neighboring atom. While scattering factors can be derived from experimental data in certain limited cases, the use of ab initio theoretical scattering phase shifts and amplitudes has greatly expanded the ability to model more complex structures and to include the effects of multiple scattering. While the calculations are complex and the results of these calculations still cannot match the accuracy of high-quality experimental EXAFS measurements, they have been demonstrated to accurately model EXAFS in a great number of systems over the past 30 years. Here, some of the finer points of the uses and limitations of applying theoretical scattering factors to the analysis of experimental EXAFS spectra are discussed.