In this chapter the authors discuss the mechanistic and other details of a few industrial carbonylation processes. These are carbonylation of methanol to acetic acid, methyl acetate to acetic anhydride, propyne to methyl methacrylate, and benzyl chloride to phenyl acetic acid. The authors also discuss two other catalytic reactions that involve CO as one of the reactants. These are the water-gas shift and Fischer–Tropsch reactions. Many of the fundamental concepts discussed in this chapter are neatly illustrated by the methanol carbonylation reactions. The chapter concludes with a question–and–answer section.