Mass spectrometry is a key technology in proteome research. Our research group focuses on the elucidation of functionally important proteins and their post-translational modifications using electrospray ionisation (ESI) and matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry in combination with miniaturized separation techniques and stable isotope labelling. These methods allowed us to study protein-protein interactions which e.g. play important roles in T-cell signalling and the assembly of spliceosomes.