The use of polymer-supported reactants in organic synthesis is currently of considerable interest, especially in the context of combinatorial syntheses. To carry out successfully reactions using polymer-supported reactants it is important to be aware of what takes place inside the beads. Examples are presented in this article which show that compared to the analogous homogeneous reactions systems, polymer-supported reactions can show substrate selectivity, be slower or faster, follow a different reaction course, or give a significantly different stereochemical result.