Carbon monoxide is one of the most important C1 molecules in organic chemistry. Many novel procedures for its conversion have been developed, and some have even been industrialized. In this review, we discuss and categorize CO chemistry into four classes: (1) transition-metal-mediated carbonylation, (2) strong-acid-initiated cationic carbonylation, (3) anionic carbonylation, and (4) free-radical carbonylation. Relevant achievements are selected and discussed in detail.