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Protein post-translational modifications (PTM) increase the functional diversity of the proteome by the covalent addition of functional groups or proteins, proteolytic cleavage of regulatory subunits or degradation of entire proteins. These modifications include phosphorylation, glycosylation, ubiquitination, nitrosylation, methylation, acetylation, lipidation and proteolysis and influence almost all aspects of normal cell biology and pathogenesis. Therefore, identifying and understanding PTMs is critical in the study of cell biology and disease treatment and prevention. In addition to PTMs, there are other artefactual protein modifications that are added due to the experimental protocol followed by the researchers. Some examples are carbamydomethylation or oxidation. The proteomics community has developed tow major resources for protein modifications (including PTMs): Unimod and PSI-MOD. However, modification idenfifiers from these two resources are not trivial to map since some of the modification in Unimod are not present in PSI-MOD and vice versa. Also, every search engine uses their notation and either Unimod or PSI-MOD. The PRIDE Modification library is used to retrieve the modification information for a specific identifier from different databases: Unimod, PSI-MOD and the PRIDE Modification controlled vocabulary (internal nomenclature used in PRIDE tools). This library is now used by different tools and pipelines.