Behavior is controlled by complex network of neurons located in the brain. It has been impossible to study neural regulatory mechanism of behavior since there is no technique to control the activity of specifi c type of neurons in vivo with high time accuracy. Recently, new technique called “optogenetics” is developed. Optogenetics enable control the activity of specifi c type of neurons by illuminating light which is less inevasible and permeable by expressing light activated molecules. To achieve optogenetics many knowledge and technique were required, such as molecular biology, physiology, electrophysiology, genetic engineering and photonics. However, it become much easier to introduce optogenetics since many equipments specialized for optogenetics are commercially available. This article explains outline of manipulation of orexin neural activity involved in sleep/wakefulness in vivo using optogenetics.