Publisher Summary This chapter reviews the usage of the numerical method of analysis that is used to model the concentration-time profiles of various reactions in a batch reactor. The chapter also provides sizing of the batch, semi-batch, continuous flow stirred tank, and the plug flow reactors for both isothermal and adiabatic conditions. Reactor design is primarily concerned with the type and size of the reactor and its method of operation for the required process. The design of any process where chemical reaction is required is given due consideration. The thermodynamics involving the equilibrium of a reaction are investigated before the chemical kinetics in relation to the reaction mechanisms. There are various advantages of using the liquid phase rather than the gas phase operation, if for the desired product the reactor is smaller. This is because the physical properties of liquids are greater than those for gases, for instance the heat capacities and the thermal conductivities, factors that increase the heat transfer. Also, the small equipment size results in lower power requirements and capital costs. The main disadvantages are corrosion and catalyst losses. In considering a liquid system, all operating conditions must fall within the two-phase region.