Electroencephalography is the process of measuring Brain Waves by measuring minute electrical voltages along the outside of the scalp. It is a non-invasive method and is the most common data aquisition apparatus for Consumer Oriented Brain-Computer Interface devices. The various positive and negative characteristics of EEG as the operating principle of a device are listed below.
The electrical nature of the brain is a result of the interactions between its neurons. In a simple representation, neurons interact via the transfer of charged ions to each other. Each of these interactions is far too minute to be detected by an electrode on the scalp, what is detected is the net sum of all of the activity in an area.  This activity can then be split into the various brain waves using mathematical filters, based on previous knowledge and research that was conducted using more accurate and complex data-acquisition apparatus.
- Low-Cost (Neurosky Mindwave available for under $100). 
- Non-invasive, consumers do not want a surgically implanted prothetic for what is in some ways just a toy.
- Easy to use, implemented in an ergonomic headset design
- Well understood from its long history in the medical field
- Quality of data is low compared to semi-Invasive (ECG) and invasive (Neuroprosthesis) methods.
- As a result, limited output channels (with bad data, trying to use too many channels will lead to overlap of commands and decrease in reliability of BCI).
- Accuracy of raw data with regards to actual nature of signals is disputed (separate to low quality of data, as training can hide this problem for gaming/control purposes, however for any diagnostic application, a device such as that pictured device is completely useless).