The particular focus of the present book is the application of mass spectrometry to the investigation and characterization of noncovalent biomolecular complexes. Both electrospray ionization and matrix-assisted laser desorption/ionization methods are discussed. Electrospray ionization mass spectrometry is particularly effective for studying noncovalent interactions because the sample is introduced from an homogeneous solution which can be maintained under near physiological conditions of pH, concentration and temperature. Mass spectra of large complexes (several hundred kDa) have been obtained using sector, quadrupole, time-of-flight and ion cyclotron resonance mass spectrometry. The use of other techniques for analyzing supramolecular complexes is also presented, particularly in combination with mass spectrometry. These include such emerging techniques as surface plasmon resonance as well as more established methods, such as X-ray crystallography and NMR.