This lecture aims at developing facility with devising plausible mechanisms for acid- and base-catalyzed reactions of carbonyl compounds, carboxylic acids, and their derivatives. When steric hindrance inhibits the A/D mechanism of Fischer esterification, an acid-catalyzed D/A mechanism can still occur. Substituent influence on the equilibrium constants for carbonyl hydration demonstrates four effects: bond strength, steric, electron withdrawal, and conjugation. Cyclic acetals play an important role in protecting the carbonyl groups of sugars, but acetals also can be used to protect alcohols, as can silyl ethers. Using amines instead of alcohols allows converting carbonyl compounds to imines via carbinolamines.