An overview of the calculation o photoionization cross sections for H&ngr;<1 keV is presented with partiular emphasis on providing a guide for evaluating the accuracy of theoretical work. An attempt is made to focus upon the various approximations made in each level of calculation and point out in which ranges they should be good. Central‐field, Hartree‐Fock, and more sophisticated methods are reviewed, particularly as they apply to free atoms and ions. In addition, the striking similarity of such calculated cross sections to core level photoemission in solids is pointed out.