The main target of this review article is to try to find the reasons for the drastic difference between expected and observed mechanical properties of polymer nanocomposites prepared via blending the matrix and the nano-reinforcement. Additional target is to recommend thereafter ways for solving of this problem. Based on the published materials the conclusion is drawn that the main reason for this discrepancy is the poor dispersion resulting in formation of particles with sizes in the micrometer but not in nanometre range. For this reason, it is assumed further that these nanocomposites hardly exist. Since currently are missing techniques and instrumentation for a proper dispersion of the reinforcement to single nanoparticles, it is recommended to avoid the dispersion step during manufacturing of polymer nanocomposites. Two techniques are described for this purpose, representing application of the rather new concept of “converting instead of adding” for preparation of polymer nanocomposites. Copyright © 2018 VBRI Press.