After the long and strenuous efforts covering more than 50 years and the tortuous experiences, feasibility of the scramjet concept has finally been proven. In this paper, the main factors influencing the technical maturity of the scramjet engine are briefly analysed. A matter of utmost concern for this new type of air-breathing engine is the net thrust. The production of engine thrust using supersonic combustion encountered a number of practical requirements which were often found to contradict each other. Several flight tests showed that the net engine thrust was still not as good as expected. The acceleration capability and mode transition of scramjet with liquid hydrocarbon fuels (kerosene) operating at flight Mach numbers about 5 has become the bottleneck preventing scramjet engine from continuing development. Research showed that the use of endothermic hydrocarbon fuels is not only necessary for engine cooling but also a critical measure for improving engine thrust and performance. Changes of thermo-physical-chemical characteristics of endothermic fuels during heat absorption make additional contributions to the combustion performance which is essential to the scramjet net thrust. Currently, the technology of experimental simulation and measurement is still lagging behind the needs. The complete duplication or true similarity of atmospheric flight environment, engine size and test duration remains impossible. Therefore, computational fluid dynamics (CFD) has become an important tool besides experiment. However, numerical simulation of supersonic combustion encountered challenges which come from both turbulence and chemical kinetics as well as their interaction. It will inevitably affect the proper assessment of the engine performance. Several frontiers of research in this developing field are pointed out: mode transition in the dual-mode scramjet, active cooling by endothermic hydrocarbon fuel with catalytic cracking coupled with supersonic combustion, combustion stability, experimental simulation and development of test facilities, measurements of the inner flow-field characteristics and engine performance, turbulence modeling, kerosene surrogate fuels and reduced chemical kinetic mechanisms, and so on. Also, directions for future research efforts are proposed and suggestions for the next 5-10 years are given.