Amino acid composition is usually amongst the first data to become available for an unknown or unidentified protein. Structural proteins often show compositional bias - for example, their composition may be heavily dominated by a single amino acid - but the implications of this are seldom clear because most discussions of such proteins are organized along different lines - usually in terms of source, secondary/tertiary structure, or physiological function. Instead, this series of review articles classes structural proteins on the basis of individual residues that are strikingly over-represented, whether they dominate the composition or not, and examines the resulting groups in terms of structure and function. This first article provides an overview of the amino acid modifications, cross-links and secondary structures common in structural proteins, and introduces the compositional categories that will be examined. It then focuses on the first two categories: proteins rich in glycine, including elastin, resilin, silks, keratins, plant Gly-rich proteins, and mollusc shell proteins, and proteins rich in proline and/or 4-hydroxyproline (Hyp), including collagen, marine mussel Mefp-1, and plant Hyp-rich glycoproteins.