Brain–computer interfaces (BCIs) employ neurophysiological signals derived from the brain to control computers or external devices. By enhancing or replacing human peripheral functioning capacity, BCIs offer supplementary degrees of freedom, significantly improving individuals’ quality of life, particularly offering hope for those with locked-in syndrome (LIS). Moreover, BCI applications have expanded across medical and nonmedical domains, including rehabilitation, clinical diagnosis, cognitive and affective computing, and gaming. Over the past decades, with a wealth of brain signals captured invasively or noninvasively, BCI has made spectacular progress. However, this also poses new challenges for signal processing techniques, such as characterization and classification. In this review, we first introduce signal enhancement and characterization methods to mine inherent patterns of nonstationary and time-varying brain signals. Then, we highlight widely adopted classification methods in BCI and the challenges they face. This article aims to comprehensively overview crucial signal processing techniques in BCI and provide suggestions for future directions.