The flammability limits are discussed in this chapter. These limits are of fundamental importance for premixed flame theory and of practical importance for industrial applications. The presentation starts by analyzing the condition for the existence of the flammability limits, which is related to a minimum laminar flame velocity. Afterwards, the chapter presents the derivation of the steady-state one-dimensional theory with one-step kinetic reaction, developed by Spalding for the flammability limits and flame quenching. Spalding’s theory introduces a critical condition that defines the flammability limits. The theoretical approach that identifies the flammability limits as a direct competition between chain branching and chain termination reactions is also presented. The chapter also discusses a more complex theoretical approach, based on detailed chemical kinetic mechanisms and the heat loss by thermal radiation. Moreover, an analysis is provided to account for the effect of initial parameters, such as temperature and pressure, and the effect of adding a diluent or another fuel to the flammable mixture. Finally, some estimation methods are presented.