There are several reasons for the success of the solid-phase approach to peptide synthesis. The first is the ease of the procedure, the acceleration of the overall process, and the ability to achieve good yields of purified products. The second was the unanticipated discovery of many new biologically active peptides and the expanded need for synthetic peptides to help solve problems in virtually all disciplines of biology. In many cases, the solid-phase technique has been the method of choice. This approach, of course, does not replace the classic solution synthesis methods, but rather supplements them. The choice of techniques depends on the objectives of the synthesis. When carefully worked out, the solution methods can give high yields of highly purified products in large quantities. Many superb syntheses of active peptides have been achieved in this way. The solid-phase method has also yielded many large active peptides. It is particularly useful when large numbers of analogs, in relatively small quantities, are required as in structure-function studies on hormones, growth factors, antibiotics, and other biologically active peptides or for determining the antigenic epitopes of proteins. In addition, it has on occasion been scaled up for production of kilogram quantities. One of the unique uses of solid-phase synthesis has been the synthesis of peptide libraries. Most of the work on this new field in which thousands or millions of peptides are prepared simultaneously has been by solid-phase methods. This new technique is proving to be of great practical importance in rapid drug discovery of peptide, peptide mimetic, and nonpeptide compounds. Developments in screening methods now allow the examination of large numbers of compounds, and active products with structures unpredictable from natural product sequences are being found in this way. The properties of the solid-phase system, the changes in the chemistry, and the applications of the technique to biological problems are discussed in detail in subsequent articles of this volume.