The reactions of lignins during pulping and bleaching are divided into two categories, i.e. nucleophilic additions and displacements, involved in pulping processes, in later phases of lignin-degrading bleaching and in lignin-retaining bleaching, and electrophilic additions and displacements, initiating lignin-degrading bleaching processes. The attack by nucleophiles, present in pulping liquors, takes place at carbonyl carbon atoms or at vinylogous carbon atoms in intermediates of the enone type, in particular in quinone methides. In addition, nucleophilic groups in the alpha-(or gamma-) position of the side chain attack the beta-carbon atom in a neighbouring group participation type of reaction which, in beta-aryl ether structures, leads to fragmentation. The initial attack by electrophiles, present in bleaching liquors, takes place at sites on the aromatic rings and side chains which are activated by free or etherified phenolic hydroxyl groups. This general concept is illustrated by numerous examples of reactions, known to be involved in pulping and bleaching processes. Possible applications of the concept in the section of appropriate pulping and bleaching combinations and sequences are indicated.