Electrospray ionization mass spectrometry is an easy, rapid method for the verification of proper peptide synthesis and for the identification of most synthetic by-products. A synthesis-purification scheme has been described that uses mass analysis to (1) confirm the presence of the proper product in the crude peptide mixture, (2) guide the purification process, and (3) confirm the mass and purity of the final product. Even though many of these steps could be performed just as well with other ionization techniques, the liquid-flow characteristics of electrospray source are clearly an advantage when LC-MS is required. In addition, the ease with which fragment ions can be generated to provide structural information, even with the least sophisticated instruments, is a further advantage of ESI-MS. Although much of the operation described here was done manually, many of the steps could be automated with little additional effort (e.g., use of an autosampler). Quadrupole and ion trap instruments are widely available at present and provide the chemist with a variety of instruments from which to choose. Electrospray time-of-flight instruments will be commercially have just become available and should also provide similar results. As electrospray instruments continue to evolve, the instruments display greater performance and enhanced user-friendly interfaces, yet are lower in price and smaller in size. These features should lead to even more widespread use for the characterization of synthetic peptides.