Mass spectrometry allows rapid chemical analysis of untreated samples in the ambient environment. This is a result of recent rapid progress in ambient ionization techniques. The most widely studied of these new methods, desorption electrospray ionization (DESI), uses fast-moving solvent droplets to extract analytes from surfaces and propel the resulting secondary microdroplets towards the mass analyzer. This review of DESI and other ambient methods centers on the accompanying chemical processes. Manipulation of the chemistry accompanying ambient ionization can be used to optimize chemical analysis, including molecular imaging. Solvent effects, geometry effects, electrochemical processes and mechanisms are covered. Extensions of the methodology to solution-phase analysis, to stand-off detection and to therapeutic drug analysis using miniature mass spectrometers are also treated.