Spider silks have astounding mechanical properties. In fact, dragline silk has greater tensile strength than commonly used synthetic materials such as nylon filament and capture spiral silk is among the most elastic protein known. The recent cloning of spider silk genes has revealed that silk proteins are composed of tandem arrayed ensembles of a small number of amino-acid sequence motifs. These repetitive motifs form the structural modules within silk fibers, and ae critical for determining the mechanical attributes of the silk. In this chapter, I examine the evolution of these motifs in the 11 published spider silk gene sequences. Extensive rearrangements of the motifs have occurred among the orthologous and paralogous proteins. Phylogenetic analyses suggest that numerous length mutations and recombination events have taken place in orthologous genes from closely related species and even within sets of alleles from the same species. Such genetic events appear to be critical for the homogenization of amino acid repeats within the silk proteins. The characterization of additional silk genes will clarify the relationships among novel amino acid motifs, the homogenization of motifs within a protein, and the function of silk fibers.