Publisher Summary This chapter discusses the importance and usage of non-isothermal reactors in industrial applications. The chemical reactions depend on temperature, either absorbing (endothermic) or generating (exothermic) a large amount of heat. Industrial reactors often operate under the non-isothermal conditions. In such conditions, the temperature and the composition vary from point to point. Either a heating/ cooling coil or an external heat exchanger can affect the temperature. Some reaction rates double for a rise in temperature of 10–15°C, and temperature affects the properties of the reaction mixture such as the density, specific heat, thermal conductivity and viscosity, enthalpy, mixing patterns, and the energy for efficient mixing. Temperature coefficients are not as great as for the rate constant; therefore, averaged values of the physical properties are often used for design purposes. Some reactors operate under adiabatic conditions, where there is negligible heat input to the reactor. The design of the non-isothermal reactors involves the simultaneous solution of both mass and energy balances. In reactors some such as the packed bed or fluidized catalytic cracking unit (FCCU), the momentum balance is considered with both the mass and energy balances. This chapter also discusses autothermal reactors, the conversion of ammonia, and finally, the optimum-temperature progression, with derived equations for reversible exothermic reactions.