Brain-computer interfaces (BCIs) allow direct communication between the brain and external devices. There are invasive BCIs that are implanted in the brain, partially invasive BCIs implanted in the skull, and non-invasive BCIs that record brain activity from the scalp using electroencephalography (EEG). EEG measures voltage differences between neurons which are amplified, filtered, and interpreted by a computer program. BCIs have applications in areas like criminal investigations, home automation, airplane control, and helping people with disabilities communicate. While BCIs open new possibilities, challenges remain in interpreting complex brain signals and developing portable equipment.