In the preceding chapter we have discussed relatively simple facts about the nature and performances of microdetection by use of ion exchangers. Solid ion exchangers are known to possess the capacity to adsorb a variety of inorganic and organic species from aqueous and nonaqueous solutions. However, this property will not be in the focus of our interest. Neither will be discussed the use of ion exchangers in classical column separations and batch preconcentrations, nor in gas-liquid and liquid-liquid chromatography. The purpose of this chapter is to survey the present-day knowledge concerning: (1) the principles of analysis using a reactive polymer or some other sorbent as a direct medium for microdetection, (2) the state of analyte-reagent system in solution compared to the state in the presence of a few resin beads or finely divided ion exchanger suspension, and (3) the sytems applied to quantitative analysis by solid-state spectrometry.