Movement and motor imagery are accompanied by changes in rhythmic activity recorded over the sensorimotor cortex. Decreases and increases in these sensorimotor rhythms (SMRs) are referred as event-related desynchronization and event-related synchronization, respectively. These changes are typically somatotopically localized. Over the past twenty years, many studies have shown that SMR changes associated with motor imagery can serve as useful control signals for brain-computer interfaces (BCIs). This chapter discusses BCI usage of SMRs. It begins by reviewing their nature and basic characteristics. It then addresses the issues critical for recording and analyzing them, and reviews their existing and possible future BCI applications.