Reality alteration was the result of a spell or ritual in which the very nature of a dimension was altered totally or partially, affecting space-time as well as the memories and perceptions of a dimension's inhabitants. Only beings with a heightened sense of self-awareness, like Adam, or beings "outside reality", like the mentally ill, were capable of perceiving such changes.
- In Sunnydale 1997, Billy Palmer manifested as an astral body after he was beaten into a coma. Along with his consciousness, the nightmares of the people of Sunnydale manifested and modified reality; Giles speculated that this was due to the unnatural energies of the Hellmouth causing Billy to bring the nightmare world with him when he returned to this world. Willow accompanied a tenor on stage as a soprano to perform Madame Butterfly, Xander was stripped down to his underwear in the middle of entering a classroom and was later pursued by a evil clown, Giles forgot how to read, Cordelia found herself in unappealing clothes and uncombed hair before being forced to participate in the chess club, and Buffy was rejected by her father before being buried alive by The Master, only to rise as a vampire. This world was undone when Billy witnessed Buffy defeat his own nightmare- the subconscious manifestation of his fear of his softball coach who beat him after he lost the game- and accepted the need to wake up.
- In 1998, a ghost of James Stanley altered reality by bringing a green monstrous arm rise out of a locker to stack Xander, turning kraut dogs and spaghetti into snakes and summoning wasps to the sociology.
- In 1998, Anyanka granted a wish to Cordelia Chase, creating an alternate timeline in which Buffy Summers never moved to Sunnydale, causing the town to be taken over by the Order of Aurelius. This alternate reality was undone when Giles summoned Anyanka to him and destroyed her amulet.
- In 1999, Willow cast a spell to have her will done in an attempt to get over the pain of Oz's departure, but the exact wording she used caused anything she said to come true, modifying reality, though only when she did so inadvertently. With this ability, she rendered Giles completely blind, made Xander a literal demon magnet, and caused Buffy and Spike to become engaged, until she realized what she had done and cancelled the spell.
- In 2000, Jonathan Levinson performed an Augmentation Spell in order to make himself more popular. As a result, he became the best of everything, the ideal man. He became a seducer, author, actor, inventor, musician, military consultant and demon hunter. The spell, however, created an opposing force of evil, everyone's nightmare. Buffy caught on to the spell when she realized that Jonathan couldn't possibly have done everything he was reputed to have done if he had never left Sunnydale.
- Months later, monks of the Order of Dagon performed a reality alteration ritual to give human form to The Key, effectively creating Dawn Summers. They also altered reality, as well as the memories of the Slayer, her family and friends, as well as all others who had ever been associated with Buffy Summers, to make Dawn fit into this new reality as if she had been naturally born. Dawn began to fade from existence after the destruction of the Seed of Wonder, but she was restored when Buffy, Willow, and Illyria brought magic back.
- In 2001, Skip altered reality so that Cordelia Chase had become a famous and successful actress instead of a member of Angel Investigations. In this reality, Wesley Wyndam-Pryce had lost an arm due to a Kungai demon incident, and Allen Francis Doyle had passed his visions on to Angel, who, because of the stress of the power and the absence of Cordelia's humanizing influence, was driven insane. When Cordelia- who still had some awareness of the old reality- kissed Angel to take the visions from him, Skip restored the original history and gave Cordelia the alternative of becoming part-demon.
- In 2002, Jasmine performed a spell to remove every record regarding the Beast, as well as wipe any memories of him, from this reality. The only person unaffected was Angelus, who, having been cursed, did not technically exist when the spell was cast, as well as sources of information acquired from other dimensions after the spell was cast.
- In 2003, the demon Cyvus Vail, acting on behalf of Wolfram & Hart, led a team of sorcerers and wizards in order to give Connor, son of Angel, a new, normal life with a human family, and removing every memory related to him from the memories of the Angel Investigations staff. Only Angel, Cordelia Chase, Eve Levin, Sahjhan, the Senior Partners and Vail kept memories of the unaltered reality, although Wesley and Illyria's memories of the old world were restored when Wesley destroyed an Orlon Window containing recollections of the true history while he, Illyria and Angel were in the same room. The alteration was undone when Buffy destroyed the Seed of Wonder, erasing the memories Connor's new family had of him and editing his own recollections so that he now recalled his new life in the same sense that someone would remember watching a film, although it would appear that the actual legal paperwork for Connor's new life was still intact.
- After the Senior Partners turned back time to undo Los Angeles being sent to Hell in order to restore Angel to life, reality was altered so that the Los Angeles branch of Wolfram & Hart had never legally existed, with the location now the future site of a Doublemeat Palace and all land records showing no trace of the firm's presence, even as Angel and his friends all remembered their activities.