Reversible Hydrocolloids impressions are the most simple, easy and reliable of all the available impression technics, when their indications are respected. The precision of this technic, although excellent, can not render this technic universal. The indications and limitations of this technic are directly related to the properties of the reversible hydrocolloids. Good clinical results will depend on: a very good gingival retraction technic (with enough space for the material) and the immediate casting of the impression; the knowledge of the intrinsic qualities and performances of the material during all the clinical steps, without trying to surpass or betray those qualities.