The de novo design of alpha t alpha, a helical hairpin peptide, is described, alpha t alpha (alpha-helix/turn/alpha-helix) was developed to provide a model system for protein folding at the level of secondary structure association and stabilization. According to the prevailing models of protein folding, the second step in the folding process is the association and stabilization of secondary structural elements or microdomains. A brief description of the design, along with CD and NMR evidence confirming the conformation of the peptide in solution, has been published (Fezoui Y, Weaver DL, Osterhout JJ, 1994, Proc Natl Acad Sci USA 91:3675-3679). The present work includes a full description of the design process, including the trade-offs that were made during the development of the peptide, a discussion of recent experimental results that were not available at the time of the original design, indications of areas where, in retrospect, the design might have been done differently, and a discussion of how the present work fits into the field of de novo protein design.