Research on Janus nanoparticles has been thriving over the past few years, focusing both on novel preparation strategies and on investigations of their unique properties. In this paper we review the main contributions to this field reported in the literature, dividing Janus nanoparticles into three main categories, depending on the route followed for their preparation. The first group of Janus nanoparticles is those obtained via self-assembly, of, e.g., block copolymers, and mixtures of ligands that in some cases show competitive adsorption on the surfaces of the nanoparticles. The second group comprises Janus nanoparticles obtained through a masking step, in which particles are trapped at the interface between two phases, so that a modification to the particle surface is made only on one side. Preparation of the third group of Janus nanoparticles relies on the phase separation of two different substances, usually either two polymers, or a polymer and an inorganic material. The peculiar properties of Janus nanoparticles, derived from their asymmetric structure, allow for their controlled self-assembly and surface activity. As a result of the simultaneous presence of two different regions in Janus nanoparticles, which can be designed to have different hydrophobicity and thereby mimic the behavior of surfactants, they can form stable clusters with defined size, and substantially reduce the interfacial tension between two different phases. Additionally, Janus nanoparticles can bring together different materials in a segregated manner at the nanoscale, thus combining widely different properties in single entity, as in the case of heterodimers. (C) 2011 Elsevier Ltd. All rights reserved.