The objective in this chapter is to introduce the principles and methods of electrospray mass spectrometry (ESMS), a relatively new technique for the analysis of complex, polar, and labile molecules. Based on electrospray (ES) ionization at atmospheric pressure of solute species in volatile solvents, ESMS has attracted widespread interest only since 1988, when it was shown capable of producing and weighing intact ions of very large biomolecules (1). Along with laser desorption (LD) (2–5), ES has now become an ionization method of choice for MS analysis of molecules in the mass range above 10 kDa. It is also finding increasing application in the analysis of smaller species and seems destined to become an indispensable tool in the emerging discipline of biological mass spectrometry (6). In this account we describe the essential features of ESMS in an attempt to provide some perspectives on how it works and how it can be used. The interested reader is referred to several recent reviews (7–10) for additional information on results that have been obtained, comparisons with other methods, and references to the literature.