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 importanceRole of aromatic amino acid clusters in protein-protein and protein-drug interactionsCell-penetrating peptides and Elastin-like peptidesUnusual phenomenon of fluorescence observed with repeats of some non-aromatic amino acidsThe use of residue cluster classes to represent protein structure and model structure-function relationships.