This chapter introduces the filed of brain-computer interfaces (BCI), also called brain-machine interfaces (BMI), which has seen impressive achievements over the past few years. A BCI monitors the user’s brain activity, extracts specific features from the brain signals that reflect the intent of the subject, and translates them into actions. BCI technology offers a natural way to augment human capabilities by providing a new interaction link with the outside world and, thus, it is particularly relevant as an aid for patients with severe neuromuscular disabilities. The chapter will first review the different kind of brain signals that can be recorded as input for a BCI. Then, it will discuss a series of principles to build efficient BCIs that are independent of the particular signal of choice. It will follow a short overview of BCI attempts to improve the outcome of neurorehabilitation, especially for motor control in stroke patients. The chapter will conclude pinpointing some future research directions in the field of BCI.