The human brain is perhaps the most fascinating and complex signal processing machine in existence. It is capable of transducing a variety of environmental signals (the senses, including taste, touch, smell, sound, and sight) and extracting information from these disparate signal streams, ultimately fusing this information to enable behavior, cognition, and action. What is perhaps surprising is that the basic signal processing elements of the brain, i.e., neurons, transmit information at a relatively slow rate compared to transistors, switching about 106 times slower in fact. The brain has the advantage of having a tremendous number of neurons, all operating in parallel, and a highly distributed memory system of synapses (over 100 trillion in the cerebral cortex) and thus its signal processing capabilities may largely arise from its unique architecture. These facts have inspired a great deal of study of the brain from a signal processing perspective. Recently, scientists and engineers have focused on developing means in which to directly interface with the brain, essentially measuring neural signals and decoding them to augment and emulate behavior. This research area has been termed brain computer interfaces and is the topic of this issue of IEEE Signal Processing Magazine.