Abstract: In 1954, Felix Wankel became the first in the world to successfully develop a rotary engine with the cooperation of the NSU. The invention of the rotary engine is a revolution in the structure of the internal combustion engine. The rotary engine exhibits advantages of simple structure, small and light, stable operation, and better high-speed performance. However, the development of the rotary engine is seriously restricted by its poor sealing problem. The apex seal is provided on each apex of the rotor to keep each working chamber gas-tight. Since it is directly exposed to high-pressure combustion gas and subjected to various kinds of restraint involved in the planetary motion, most efforts have been concentrated on the study of its performance and durability. The apex seal has three key problems: vibration, gas leakage, and wear. These problems will lead to the emergence of chatter marks that are called the nail marks of the devil. These problems directly affect the working performance and service life of the rotary engine. Mazda and NSU have made great contributions to the improvement of apex seals. Many types of apex seals have been developed, such as solid-type, split-type, cross-hollow, and three-piece. Many materials have been used as apex seals, such as alumetizing carbon, special cast iron, and fiber-reinforced ceramics. With the development of material technology, some new materials and treatment processing technology could be applied to the apex seal, e.g., laser surface treatment and new coatings. Carbon fibers, graphene, and nanometer materials could improve the wear resistance and mechanical property. This paper summarized the achievements of Mazda, NSU, and Curtiss-Wright. Finally, according to the new materials and treatment processing technology, the new structures and new materials for the apex seals were given in this paper. The trumpet-shaped notch structure, roller pin structure, cantilever flexure structure and multi apex sealing system can be used to improve the sealing performance of apex seals. Laser surface textured and new materials such as nano-ceramic and graphene are the breakthrough point to solve the sealing and wear problem of apex seals. This paper puts forward some suggestions for the future development of apex seals.