The choice of materials for fillings in primary teeth depends largely on the possibility of establishing good cooperation with the child, the incidence of dental caries and caries risk. An important feature of dental materials for fillings in the primary dentition is their deterrent effect, as well as the possibility of good adhesion to the cavity walls, the ability to seal the closure and insensitivity to moisture and certainly the most important property, biocompatibility. GICs are the materials of choice to be applied whenever there is a difficulty to achieve a moisture control. They are indicated in young children, patients with high caries risk, especially in patients with special needs and when it is difficult to perform dental treatment. Heating or thermo-curing of GICs "on command" enhance the power of GIC adhesion to enamel, reduce marginal leakage, and therefore it is recommended during initial phase of material setting. Chemical bond to enamel is based on the interaction of the carboxyl groups of polyacrylic acid and calcium hydroxylapatite from enamel. Because of the possibility of continuous release and accumulation of fluoride, GICs have a preventive effect. For all materials which are in close contact with the body biocompatibility is essential. GICs are classified as highly biocompatible materials, and deserve a special place in the dental materials group. Resin-modified GICs cannot be considered as biocompatible as conventional GICs. Dental personnel may be at risk of an adverse effect of resin such as contact dermatitis.