Criteria are given to characterize mature theories in contradistinction to developing theories. We lean heavily on the physical sciences. An established theory is defined as a mature one with known validity limits. The approximate truth of such theories is thereby given a quantitative character. Superseding theories do not falsify established theories because the latter are protected by their validity limits. This view of scientific realism leads to ontological levels and cumulativity of knowledge. It is applied to a defense of realism against recent attacks by Laudan.