Post-translational modifications serve many different purposes in several cellular processes such as gene expression, protein folding and transport to appropriate cell compartment, protein-lipid and protein-protein interactions, enzyme regulation, signal transduction, cell proliferation and differentiation, protein stability, recycling and degradation. Although several-hundred different modifications are known, the significance of many of them remains unknown. The enormous versatility of the modifications which frequently alter the physico-chemical properties of the respective proteins represents an extraordinary challenge in understanding their physiological role. Since essential cellular functions are regulated by protein modifications, an improvement of current understanding of their meaning might allow new avenues to prevent and/or alleviate human and animal diseases.