Turbofan engine technology has been widely used for propulsion of commercial airplanes in the subsonic range. Since the early 70's, turbofan's efficiency has greatly evolved, driven firstly by the core (thermodynamic) efficiency improvement, followed by a continuous increase in the bypass ratio (BPR), i.e. the rate between the engine fan airflow and the engine core airflow mass, with a focus on propulsive efficiency improvement. The core efficiency strategy, strongly supported on the engines' thermodynamic properties boost, i.e. using higher overall pressure ratios (OPR) and turbine inlet temperatures (TIT), has contributed decisively to the engine's thermodynamic efficiency improvement. This pathway, which required important design solutions, like engine air bleed for blade and case cooling, blade materials & coating technologies for high temperature engine components and blade tips & core clearance control, has already reached a premium performance, in which further improvement will be residual, i.e. further engine efficiency improvements must rely on other technological strategies. In parallel, the use of higher BPR, which a focus on the reduction of exhaust airflow speed and the increase in airflow mass, i.e. a lower Fan Pressure Ratio (FPR), has allowed the improvement of propulsive efficiency, hence, contributing to the overall engine's efficiency improvement. In this context, the low BPR turbofan (up to 2:1), initially used in the 70's, for commercial airliners, has evolved for the medium BPR (up to 4:1) turbofans in the 80's, large BPR (up to 8:1) in the late 90's and, finally, up to 12:1 BPR in the 2010's, have contributed to the continuous turbofan propulsive efficiency improvement. Nevertheless, the aeronautic propulsion industry, currently driven by the continuously stringent energy efficiency and environmental standards, has been encouraged to pursue additional energy efficiency and environmental performance improvements. One of the viable technological pathways, is the use of even higher BPR, with the so called Ultra High Bypass Ratio (UHBPR) engine approach, which uses BPR higher that 13:1 and can be seen as the turbofan engine efficiency frontier, to be used as a promising technology for near future airplane propulsion. Despite the propulsive efficiency improvements, the increase in the BPR sets some technological challenges in the engine design, associated with an increase in the engine's aerodynamic drag and weight (due to the larger fan&nacelle diameters), fan and core engine aerodynamics stability control, as well as aerodynamic engine and airframe & lifting devices integration issues. This paper presents an overview of the UHBPR engine technology and its inherent technological challenges, with a focus on the engine architecture and the required technological tools, such as geared fans, swept & twisted inlet vanes, variable pitch fan, variable fan nozzle area, required to circumvent the associated aerodynamic and mechanical operational limitations.