A review.Modern combinatorial libraries are characterized by the generation of numerous different, but structurally related compounds that exist discretely as static entities, under similar conditions, in a systematic manner.These combinatorial arrays can be produced in a parallel fashion, either as a library of individual compounds or as pooled mixturesIn applying a parallel format, the compounds are handled individually in separated compartments.This removes difficulties associated with compound mixtures; and it allows for every individual compound a straightforward evaluation of chem. integrity and structure-activity relation after biol. testing.The library size handled with this methodol. is more limited than in a pooling strategy.Pooled mixture formation is straightforward and less time-consuming than the synthesis of individual compoundsMore difficult is the anal. and screening of mixtures of components. By the split and mix technol., very large libraries can be formed progressively over multiple synthetic steps.