This monograph reviews all relevant technologies based on mass spectrometry that are used to study or screen biological interactions in general. Different distinct areas for analysis of bioaffinity interactions can be recognized in this regard. These areas include pre-column based ligand trapping followed by MS analysis, affinity chromatography following MS and post column on-line affinity profiling. Other methodologies relate to separately performed bioassays and (LC) MS analysis. Besides these, direct approaches are nowadays also used in several research areas and include direct MS based bioassays and native MS studies in which the latter looks at intact protein complexes in the gas phase.