The effect of surface heterogeneity on catalysis and chemisorption is discussed. Available recent data on calorimetry, as well as on adsorption isotherms and reaction kinetics, are carefully reviewed. Despite the fact that the current catalytic engineering practice is very much limited to Langmuir-Hinshelwood kinetics based on concepts of ideal surfaces, there is a number of cases when such an approach is not sufficient enough for the modeling of the kinetics of heterogeneous catalytic reactions.