Analytical characteristics of DESI are summarized. Examples of applications to small and large molecules, to in situ analysis, and to high-throughput analyses are presented. Evidence is provided for both a heterogeneous charge-transfer mechanism and a droplet pick-up mechanism of ionization. The speed, lack of the need for sample preparation, selectivity, and sensitivity of DESI are all demonstrated and discussed. Instrumentation is also discussed. Forensic applications as well as emerging areas of application including tissue imaging are given emphasis.