An overview is given of inelastic electron scattering in solids, with an emphasis on calculations and measurements of electron inelastic mean free paths (IMFPs). There is generally good agreement between IMFPs calculated from optical data and IMFPs measured by elastic-peak electron spectroscopy for electron energies greater than 100 eV. For lower energies, however, there are significant differences between these calculated IMFPs and IMFPs determined from X-ray absorption fine-structure experiments.