Sasuke is the second and youngest son of Mikoto and Fugaku Uchiha; they named him after Sasuke Sarutobi in the hopes that he would someday be just as strong a shinobi. Sasuke grew up in the shadow of his older brother, Itachi, a natural prodigy who many in the Uchiha clan and the village would constantly compare Sasuke and any of his accomplishments to Itachi. Sasuke himself adored Itachi, never passing up an opportunity to spend time with him. Although Itachi welcomed his company, letting Sasuke watch him train and taking him on adventures into the forests, Itachi in return rarely helped Sasuke himself become a better shinobi; when asked, he would often instead poke Sasuke's forehead and promise to do so some other time. Sasuke found this annoying, but didn't allow it to blemish his high opinion of his brother.
On entering the Konoha's Ninja Academy, Sasuke proved to be the standout of his class, consistently getting top grades. However, he could never meet the same milestones Itachi had set, resulting in their father paying Sasuke little attention. Aware of this neglect, Itachi, despite being increasingly busy, tried to stand in for their father by giving Sasuke the recognition he craved, at times even blackmailing Fugaku to spend time with Sasuke. As time went on, Itachi started becoming distant with and cold towards their family, culminating in a falling out with much of the Uchiha clan on their suspicion that he'd killed his best friend, Shisui Uchiha. Sasuke did not understand the reason for this, but he didn't mind the side-effect: his father began taking an interest in his development. Fugaku taught Sasuke how to perform the Great Fireball Technique, which he mastered in a week. Fugaku stated his pride in Sasuke for this accomplishment, but at the same time encouraged him not to follow in Itachi's footsteps.
After a long day of training, Sasuke returned home one night to find the streets littered with the bodies of the Uchiha. He rushed home to notify his family of this Uchiha Clan Massacre, only to find Itachi standing over the bodies of their parents. Sasuke tried to solicit help and comfort from Itachi, who responded by using Tsukuyomi on him to torment him with visions of him murdering their family. Horrified by what Itachi had done, Sasuke pleaded for an explanation, to which Itachi replied that it was to test his own power. Fearful that he would be next, Sasuke tried to run. Itachi cornered him and explained that Sasuke, as he then was, would not be worth killing. Only by becoming stronger, such as by acquiring his own Mangekyō Sharingan, could he prove a worthwhile challenge to Itachi's abilities. Before leaving, Itachi encouraged Sasuke to hate him, to desire revenge, and to gain power from that. Sasuke immediately followed through, pursuing Itachi and using his newly-awakened Sharingan to attack him. The attack failed and Sasuke passed out, but not before glimpsing Itachi crying; Sasuke would forget this had happened for many years.
Sasuke, now one of the last surviving Uchiha, was alone. He spent the first few days after the massacre wandering his family's compound, reflecting on the people who were now gone, killed by Itachi. Sasuke decided to do what Itachi had instructed and dedicated his life to vengeance, having no other interest than bringing about Itachi's death. He threw himself into his studies at the Academy, making no efforts to form friendships and ignoring all of the girls' attempts to gain his affection. One of his classmates, Naruto, disliked Sasuke's cool personality and the attention he received, and developed a one-sided rivalry in his pursuit to prove himself just as good as, if not better than, Sasuke. For his part, Sasuke thought little of Naruto and was usually annoyed by his outbursts, but would, at times, secretly smile at how hard Naruto worked because of him. Ironically, for all the attention he received, Naruto was the only person among his peers who understood Sasuke due to the painful experiences he had.