Resonance ionization spectroscopy (RIS) which is a selective multistep photoionization process using tunable pulsed lasers has provided a new sensitive method for the study of a variety of processes in chemical physics. These include the selective detection of very small numbers of atoms and molecules, thus providing very sensitive probes of trace pollutants, chemical reactions, diffusion, and density fluctuations. Also RIS proved to be far more sensitive than the conventional absorption or fluorescence methods for the study of binary atomic collisions. The extra sensitivity allows the investigation on the far wing of optically thin samples where the high density and high temperature effects (dimer absorption, three body effects, and self broadening), are essentially eliminated. In particular the method will be crucial in studies of S-S and S-D transitions.