Laser-induced fluorescence (LIF) may be used to make non-perturbing diagnoses of ions in plasma processing sources. Among the measurements possible are ion density profiles, distribution functions, convection and diffusion. Plasma processing sources such those used for etching, sputtering and thin film deposition may be diagnosed. In this review of LIF, examples of argon ion distributions and flow velocities in filament, hollow cathode and radio frequency plasma processing system are presented. (C) 2003 Elsevier Science B.V. All rights reserved.