The linear rheology of nanoparticle filled polymer (NPFP) melts has been a quite charming but controversial topic of long standing. This article reviews recent research advances to provide a general understanding of its universal appearance and underlying mechanism. This work summarizes the rheological criteria for determining the so-called liquid-to-solid transition with increasing filler content, the contradictory ideas of four kinds of time-concentration superposition principles proposed for constructing master curves of linear rheology, and a wide range of constitutive and phenomenological models focused on creating rheological contributions of the polymer, filler, and interface region from different perspectives. Controversies about microstructures of NPFPs including filler structure and chain dynamics of the matrix are briefly described. Several open questions are highlighted to outline the most likely general framework for the further investigation of the linear rheology of NPFP melts. (C) 2015 The Society of Rheology.