Studies of the degradation processes, types of tests, and measurements and analyses of substances leaching out from resin-based composite materials and glass-ionomer cements are reviewed. For both types of materials, the initial release rate rapidly decreases to a low, but nearly constant, level. For composites, various types of degradation processes have been demonstrated. Elements from filler particles and degradation products from the resin (e.g., formaldehyde) leak out. Many substances are not properly identified. It is, however, difficult for in vitro and in vivo degradation to be compared. For glass ionomers, a total disintegration of a surface layer is observed, together with a slow release of elements from the bulk. Of the elements released, fluoride is the most interesting. Marked differences have been shown between in vitro and in vivo solubility tests.