Proteins are constructed by the head-to-tail joining of amino acids, chosen from a 20-letter alphabet. The 20 natural amino acids have a common backbone, but a variable side chain or R-group. The R-groups may be large or small, charged or neutral, hydrophobic or hydrophilic, and conformationally restricted or flexible (see Figure 9.1). It is the physical properties of these R-groups that determine the diverse structures into which a given amino acid chain will fold. Broadly speaking, proteins can adopt fibrous or globular shapes. Repetitive amino acid sequences adopt elongated periodic fibrous structures, with common examples including elastin (skin), collagen (cartilage), keratin (hair), and b-fibroin (silk). This chapter focuses on globular proteins.