Many techniques are used to ionize molecules for mass spectrometry. These include electron impact (EI), photon ionization (PI), field ionization (FI), chemical ionization (CI), electrospray, Fast Atom Bombardment (FAB) and others. Indeed, some of these even have sub-headings, such as MPI (multiphoton ionization) and REMPI (resonance enhanced multiphoton ionization), etc. Understanding the ionization process is crucial to understanding the mechanisms by which mass spectral data are produced.