A brain-computer interface (BCI) can realize the communication and control between the human brain and computers or other electrical equipment by electroencephalography. It is a novel kind of human2computer interface. BCIs will be applied in rehabilitation, control, and other fields. This paper introduces the working principle of BCIs. Some key techniques to design BCIs are discussed from these aspects: signal processing, device control methods, and signal feedback. Besides, the application of BCI in rehabilitation medicine is also discussed. Finally, some main problems and future trends are pointed out. The discussions have guiding values to the design and research of BCI.