Post-translational modification of proteins is a key regulatory event in many cellular processes, including recognition, signaling, targeting and metabolism. Proteomics - a research field based on advanced protein chemistry, mass spectrometry and bioinformatics - enables the global analysis of post-translationally modified proteins. The feasibility of modification-specific proteomics is shown by recent studies of phosphoproteomes. Mass spectrometry plays a pivotal role in identification and also the complete characterization of post-translationally modified proteins.