Three fundamental issues: (1) how many negative ions exist in processing plasmas, (2) how negative ions affect the spatial distribution of charged particles, and (3) how negative ions affect the structure of the sheath, are discussed on the basis of knowledge attained in the past decade. Photodetachment techniques for measuring negative ion density are reviewed and results of measurements for low density as well as high-density plasmas containing halogenated gases and oxygen are discussed from a view point of how the negative ion/electron density ratio varies as a function of electron/feedstock gas density ratio. The effect of negative ions on the spatial distributions of charged particles is illustrated by solving plasma balance equations based on an ambipolar diffusion model for various electron attachment rate coefficients and electron densities; some comparisons of the results with other model studies are made. The structure of the sheath in electronegative plasmas recently clarified by a number of workers is illustrated and discussed by giving typical fluid model solutions in the spherical symmetry. (C) 2002 Elsevier Science B.V. All rights reserved.