The acid-base reaction is one of the most common reaction types in all of organic chemistry. Most of the reactions encountered in organic chemistry involve acid-base chemistry in one form or another. This chapter discusses acid-base reactions and reactivity of organic molecules. Bond polarization of the hydroxyl group does not completely explain the acidity of a carboxylic acid such as formic acid. When formic acid reacts with sodium amide, removal of the proton gives the resonance-stabilized formate anion as the conjugate base, with two resonance contributors. Since carboxylic acids are the most common organic acids, they will be used to examine structural variations that contribute to variations in acid strength. Differences in basicity between primary, secondary and tertiary amines can be attributed to inductive effects.