The fundamentals of ion and plasma diagnostics are introduced with some actual examples. For the purpose of elucidating the important role of a diagnostic method in establishing physics-based models, the implementation of negative ion density measurements using laser-induced photodetachment is described, and applications of the method to study negative ion containing plasmas are explained. Possibilities of using particle beams are discussed for the future applications on ions and plasma diagnostics. Advancements of particle sources, light sources, measurement electronics, and digital data processing support the development of diagnostics techniques for emerging applications of ion beams and plasmas.