Electrospray ionization (ESI) mass spectrometry (MS) has become an indispensable tool for studies on protein structure, folding, dynamics, and interactions. The ESI process generates intact and multiply protonated ions from proteins in solution. The charge state distribution of these ions provides a highly sensitive probe for the overall compactness of a protein in solution. Unfolded conformers lead to the formation of higher charge states than natively folded proteins. Due to its very gentle nature, ESI allows the transfer of intact noncovalent assemblies (protein-ligand and protein-protein complexes) into the gas phase. Thus, ESI-MS is ideally suited for monitoring coupled folding/binding events. The remarkable selectivity of this technique facilitates the observation of co-existing conformers and binding states. This review discusses mechanistic aspects of the ionization process, as well as selected examples that illustrate the use of ESI-MS for monitoring protein folding and assembly reactions. The combination of ESI-MS with on-line mixing techniques can provide mechanistic insights into processes occurring on very rapid time scales. We also address the interesting question whether biomolecular structures in the gas phase resemble those in solution. Experimental approaches involving hydrogen exchange and covalent labeling techniques are covered in an accompanying article.