Identification of antibody binding peptides may be based on the primary structure of the protein antigens used to raise the antibodies (knowledge- or sequence-based approach). This involves scanning the entire sequence of the antigen with overlapping peptides (peptide scan), which are then probed for binding to the respective antibody. If a natural protein binding partner is not known, one has to use combinatorial synthetic libraries with peptide mixtures, randomly generated chemically synthesized libraries of single individual sequences, or biologically produced libraries (e.g., phage display libraries, see Chapter “Epitope Mapping Using Phage Display Peptide Libraries”). This chapter describes chemically synthesized combinatorial, as well as randomly generated peptide libraries, collectively called de novo approaches, and their application for antibody epitope mapping.