Understanding of product-related variants, such as variants with post-translational modifications, is an important part of biopharmaceutical development. Deamidation is a common post-translational modification occurring in biopharmaceutical proteins, affecting L-asparagine (Asn) and to a lesser extent, L-glutamine (Gln) residues. The rate of deamidation reactions are influenced by factors including protein structure (primary, secondary and higher structure), temperature and pH. In the vast majority of cases, deamidation is undesirable in biopharmaceuticals, and may lead to potential changes in protein structure, function, stability and immunogenicity. Measurement and characterisation of deamidated biopharmaceutical variants may be challenging, particularly with regard to quantitation of the two L-aspartate isoforms that are created, L-aspartic acid (Asp) and isoaspartate (isoAsp). Deamidation may occur intracellularly or during biopharmaceutical manufacture and storage, and must be understood, minimised and controlled, particularly in a regulatory context. Process control strategies that have been employed to date include alterations to fermentation steps, additives to cell cultures, chromatographic separation of charge variants and protein engineering to remove deamidation-prone Asn residues. However, the impact of deamidated forms of biopharmaceuticals should also be thoroughly studied, as they may not necessarily represent deleterious changes to the function of the molecule or the quality of the final product. This mini-review provides a summary of the current understanding of the origins, control and measurement of deamidation during biopharmaceutical development. (C) 2015 Society of Chemical Industry