According to the patent laws of many countries, if a patented invention is not implemented or not fully implemented for four years from the date of application or three years from the date of approval without justifiable reasons, the competent authority may grant a compulsory license to exploit the invention according to the application, that is, the licensee may exploit the patent after paying a certain remuneration to the patentee. If two inventions are interdependent, and the implementation of one invention depends on the implementation of the other invention, but the consent of the invention patentee cannot be obtained, the patent laws of some countries stipulate that one invention patentee can be granted a compulsory license to exploit the other invention upon his application. If the patented invention is of great significance to national defense, national economy or public health, it can also be stipulated that others can be allowed to use it with compensation, or even the state can pay a certain fee to expropriate it. Chapter VI of China's Patent Law makes special provisions on compulsory licensing of patent exploitation.