Electrospray ionization mass spectrometry (ESI-MS) has demonstrated utility for the detection and study of weakly bound, noncovalent complexes, including protein interactions with inhibitors, cofactors, metal ions, carbohydrates, other peptides and proteins, enzyme-substrate pairings, and nucleic acid complexes. From the measurement of molecular mass of the intact complex and the individual binding partners, the binding stoichiometry can be derived. In many examples, the relative and absolute binding affinities can be deduced by the MS-based method. A review of the experimental principles of the method for studying noncovalent complexes, with emphasis on proteins, and the early studies that aided in the development of ESI-MS for this application are presented. Examples of protein complexes, such as the calcium-bound calmodulin-melittin complex, streptavidin homotetramer, and the enolase protein dimer are used to illustrate important features of the technique. A discussion on current and future applications of ESI-MS, such as the determination of the topology of macromolecular complexes, is provided. (Int J Mass Spectrom 200 (2000) 175-186) Β© 2000 Elsevier Science B.V.