The field of combinatorial peptide chemistry was created when the first limited peptide library was synthesized with multipin technology in 1984. Since 1990, the field of combinatorial chemistry has advanced rapidly. It has emerged as a powerful tool in the study of many biological systems. In essence, a combinatorial peptide library (numbering 104–108 different molecules) is chemically synthesized and subjected to screening for biological activity. Such screening allows for the rapid discovery of peptide ligands with biological interest. The amino acid sequence of an “active” peptide is then determined. Other biologic peptide libraries, such as those involving filamentous phage-display peptides, can typically accommodate only the 20 natural amino acids. In contrast, synthetic peptide libraries have the potential to incorporate d-amino acids and other unnatural amino acids, as well as specific secondary structures or scaffolding structures that may enhance biologic activity. In addition to amino acids, biological subunits such as monosaccharides, nucleotides, lipids, or even small organic moieties can be used. The advent of these peptide, nonpeptide oligomer, or small molecule library methods not only facilitates the drug discovery process but also provides important information for the fundamental understanding of molecular recognition. Keywords: Acceptor Molecule; Amino Acid; Combinatorial Peptide Library; Combinatorial Nonpeptide Oligomer Library; Combinatorial Small Molecule Library; Motif; Peptide