Stratification is a common phenomenon for collisional plasma of gas discharges. The nature of stratification and different types of striations (ionization waves) in glow discharges of inert gases are described in this entry. Progress associated with the discovery of moving striations in constricted discharges, advances of electron kinetics in low-temperature plasmas, and the development of computational models of gas discharges are reviewed. Kinetic analysis of electrons is shown to be important for all types of striations in rare gases. Examples of electron energy distribution functions measured with Langmuir probes and calculated using kinetic theory are presented for s-, p-, and r-striations observed in low-pressure DC discharges of rare gases.