This book revisits the well-known problem of diffusion and reaction in a catalyst slab in which a single reaction is taking place under steady state and isothermal condition. The model equation of second order nonlinear ordinary differential equation, has been developed by applying the material balance over the reactant species. The model equation has been numerically solved with the help of MATHEMATICA for different reaction order and kinetics to not only obtain the concentration profile of reactant inside the catalyst pellet but also the effectiveness factor, which plays an important role in the design of catalytic reactors. Thus obtained values of effectiveness factor have been compared with those found from various existing approximate analytical relations, and usefulness and limitations of these relations in predicting the effectiveness factor have been highlighted.