This article describes the inelastic scattering effect in surface electron spectroscopies such as XPS and AES. In order to improve the accuracy of quantitative surface analysis, it is very important to describe the attenuation rate of the electron signal due to inelastic scattering events in a solid. For this, “attenuation length (AL)” has been used for a long time to describe the attenuation rate. AL is, however, replaced by the “effective attenuation length (EAL)”, which includes the elastic scattering effect, because the signal electrons may not vary exponentially with the thickness of an overlayer film due to the elastic scattering. Then, the calculations and measurements of physical quantities such as electron inelastic mean free path (IMFP) and EAL are described.