The demands for highly sensitive and specific analytical techniques in biochemistry, molecular biology and biotechnology are met by new developments in mass spectrometry. Femto- to attomole sensitivity and mass accuracy in a low parts per million range can now be routinely obtained. Mass spectrometry, already accepted for studies of protein secondary modifications, must, in the future, be expected to be an important tool in protein studies on all levels, ranging from proteome analysis to studies of protein higher order structures and protein interaction.