An organic Rankine cycle (ORC) is simply a steam cycle in which the traditional steam is replaced by refrigerants or hydrocarbons. ORCs utilize clean energy resources: geothermal fluids, solar irradiation, ocean thermal gradient, heat from biomass combustion and waste heat from industrial thermal processes. The interest is growing over this technology. Large systems (> 400 kW) are commercially available while small scale systems are still under development. This technology is a new opportunity for industries and alternative solution for clean power generation. This paper recalls ORC principles, outlines challenges to be overcome (working fluids, small expanders design, performance map, heat exchangers integration, and project guidelines) and gives few data on the ORC market.