Genjutsu (Illusionary techniques) are a type of techniques utilised by ninja similar to ninjutsu, still requiring the use of chakra and handseals, but instead of causing physical damage, genjutsu techniques cause mental damage and produce an illusionary effect. Genjutsu is used by sending chakra into the target's brain to disrupt their senses and create an illusion. Doing this requires a direct sensory link with the target, usually eye contact. When used correctly, genjutsu can be a very deadly skill. The members of the Uchiha clan are known for their powerful genjutsu, which comes from their dojutsu kekkei genkai, the Sharingan. The Sharingan and the Mangekyo Sharingan can produce some of the most powerful illusions, such as Tsukuyomi and Kotoamatsukami. There is also another type of genjutsu called Sound Genjutsu which casts the illusion on anyone who hears the sound. Genjutsu can be used by a link with any of the five senses, but sight is most commonly used. Genjutsu affects almost all people, except the people who for some reason cannot be effected by foreign chakra entering their bodies. People would also not be affected if they block their senses (for example covering their eyes) Also, if the user has a Dojutsu, then they can easily detect genjutsu because they will see that their chakra flow is irregular.
There are multiple ways to counter genjutsu. The victim can use a jutsu which sends a spike of chakra throughout their body to dispel the illusion. Also, an ally can send their chakra into the victim to stop the genjutsu. The Sharingan can break through most illusions and the Byakugan can sense the chakra disruption but the user will have to get themself out of the illusion. A bijuu can break their jinchuriki out of genjutsu as well. Genjutsu can also be broken by using pain.
Deidara, using unknown methods, trained his left eye to be able to break to genjutsu.