Repeat amino acid sequences are important in protein structure, disorder, function and evolution. Repeats of some amino acids exist in proteomes across species.This volume highlights: Which repeat sequences have pathogenic consequences and why? Repeats which lead to surface hydrophobic clusters and their importance Role of aromatic amino acid clusters in protein-protein and protein-drug interactions Cell-penetrating peptides and Elastin-like peptides Unusual phenomenon of fluorescence observed with repeats of some non-aromatic amino acids The use of residue cluster classes to represent protein structure and model structure-function relationships.