Solid supports for peptide synthesis are selected for physical stability, to permit the rapid filtration of liquids.  Suitable supports are inert to reagents and solvents used during SPPS and allow for the attachment of the first amino acid. Swelling is of great importance because peptide synthesis takes place inside the swollen pores of the solid support.
Three primary types of solid supports are: gel-type supports, surface-type supports, and composites. Improvements to solid supports used for peptide synthesis enhance their ability to withstand the repeated use of TFA during the deprotection step of SPPS. Two primary resins are used, based on whether a C-terminal carboxylic acid or amide is desired. The Wang resin was, , the most commonly used resin for peptides with C-terminal carboxylic acids.