The inelastic mean free path (IMFP) is an index of how far an electron on average travels through a solid before losing energy.
If a monochromatic, primary beam of electrons is incident on a solid surface, the majority of incident electrons lose their energy because they interact strongly with matter, leading to plasmon excitation, electron-hole pair formation, and vibrational excitation. The intensity of the primary electrons, , is damped as a function of the distance,  , into the solid. The intensity decay can be expressed as follows:
where  is the intensity after the primary electron beam has traveled through the solid to a distance . The parameter , termed the inelastic mean free path (IMFP), is defined as the distance an electron beam can travel before its intensity decays to  of its initial value. (Note that this is equation is closely related to the Beer–Lambert law.)
The inelastic mean free path of electrons can roughly be described by a universal curve that is the same for all materials.
The knowledge of the IMFP is indispensable for several electron spectroscopy and microscopy measurements.