The aviation industry has been strongly driven by stringent safety, efficiency and environmental requirements. In the efficiency field, engine efficiency, allied with aerodynamic design improvements, have played an important role to reduce the fuel burn, given that fuel expenses are currently one of the major cost drivers of the air carriers worldwide. Since the beginning of the jet engine era, engine efficiency has evolved significantly, through thermodynamic efficiency improvements, associated mainly with increased engine pressure ratio (EPR) and turbine inlet temperature (TIT). Another important optimization driver has been the propulsive efficiency, associated with increased bypass ratios (BPR). Material and aerodynamic properties have imposed limits for thermal efficiency improvements, which ultimately might hinder further progresses. Meanwhile, propulsive efficiency - highly dependent on the BPR - is currently limited, due to the inherent increase in the nacelle weight and drag, derived from the higher BPR and its intrinsic larger fan diameter. In this context, the so called Open Rotor (OR) engine concept has been proposed to address the engine’s propulsive efficiency improvement, without the hurdles seen on the Turbofans (ducted fans), i.e. nacelle weight and drag increase. Furthermore, this engine technology has also incorporated the so called Contra Rotating (CR) concept, to recover the energy from the fan’s exhaust flow swirl, and, hence, improve further the propulsive efficiency. This engine concept has been initially proposed in the late 70's, as a product of an energy efficiency research program, to face the late 70's oil crisis. The technological concept has been developed and tested in the laboratory/wind tunnel and prototype’s operational field test campaigns (the most well known as Unducted Fan (UDF) and Propfan) along the 80's, focused on the medium range airplanes market. Albeit successfully from a propulsion and efficiency perspective, the increased noise level required further research. Moreover, the changes in the oil price scenario that followed the 90s have reduced the technology’s attractiveness for the world air carriers, which ultimately have contributed to the discontinuance of the research efforts. However, the advent of the continuously stringent efficiency (and emissions) standards, associated with a new oil price upward trend scenario, have awakened the interest in the technology. In this context, technological research institutions and engine manufacturers have relaunched the research programs, focused on the technology improvement and reliability tests, aimed on the medium term scenario, i.e. the next generation engine technology, to be used in mid range aircrafts. This work is supposed to present an Open Rotor Engine technology review, associated with a historical overview of its evolution pathway and current research initiatives, as well as the technological challenges to be addressed prior to its commercial use.