The general principle of SPPS is one of repeated cycles of coupling-wash-deprotection-wash. The free N-terminal amine of a solid- phase attached peptide is coupled to a single N-protected amino acid unit. This unit is then deprotected, revealing a new N- terminal amine to which a further amino acid may be attached. The superiority of this technique lies in the ability to perform wash cycles after each reaction, removing excess reagent with all of the growing peptide of interest remaining covalently attached to the insoluble resin which facilitates purification of the final product. In addition the ability to use an excess of amino acid and coupling reagent during each cycle of coupling permits the user to drive the reaction to completion i.e. high-yield. Peptides 20 residues in length can be synthesized comfortably, although much longer sequences (~ 100 amino acids) can be accessed with extensive optimisation.