Low-grade thermal energy recovery has attained a renewed relevance, driven by the desire to improve system efficiency and reduce the carbon footprint of power generation. Various technologies have been suggested to exploit low-temperature thermal energy sources, otherwise difficult to access using conventional power generation systems. In this paper, the authors review the most recent advances and challenges for the exploitation of low grade thermal energy resources, with particular emphasis on ORC systems, based on information gathered from the technical literature. An outline of the issues related to ORC system modeling is also presented, and some guidelines drawn to develop an effective and powerful simulation tool. As a summary conclusion of the revised models, a simulation tool of an ORC system suitable for the exploitation of low grade thermal energy is introduced. (C) 2014 Elsevier Ltd. All rights reserved.