Abstract: In most publications dealing with the use of polymer supports in organic synthesis, the support is usually designated by the symbol - and the supported reagents are often reported to be immobilized. When some other indications are given they often correspond to the type of resin (gel or macroporous), the amount of divinylbenzene (DVB) used as the crosslinking agent, the average radius of pores (calculated from the surface area), the total pore volume, and finally the capacity (amount of functional groups per gram of dry resin). Although they are useful, these indications are not enough to understand how the support may influence the course of the supported reactions. It must be understood that the performance of the supported reagents is strongly dependent not only on the structure of the support but also on its interaction with the reaction medium. This review discusses the present state-of-the-art and deals with the main issues. The first point concerns the morphology of the support (chiefly the porous texture) under the condition of utilization, i.e., in the swollen state; recent work carried out in using size-exclusion chromatography and in France using thermoporometry is briefly discussed. The second point concerns the study of the microenvironment and especially the characterization of the internal mobility and the local accessibility of the functional groups. The third point is the location of the functional group; is it possible to know something about the distribution of the functional groups, either on the surface or inside the polymer? Some recent examples are discussed. Finally, some new materials with surface location of the sites are discussed, namely surface functionalized latexes, pellicular resins and chemically modified polymer single crystals.