Brain states are patterns of neural activity that correlate with behavior and reflect underlying neural processing. Within the electroencephalography (EEG) recording, brain states can be classified by observing the temporary patterns or fluctuations within the electrical signal over time that can be observed across individuals. Technological advancements in recording and analysis methods have greatly enhanced our ability to study brain dynamics, which has in turn given rise to a better understanding of the structure and function of the brain. In this chapter, we define brain states in the context of EEG and provide examples of research fields studying brain states. We highlight how classification of brain states within specific applications has helped us to understand the function of both the healthy brain and the disruption of function in disease.