UFC 121: Lesnar vs. Velasquez was a mixed martial arts event held by the Ultimate Fighting Championship on October 23, 2010 at the Honda Center in Anaheim, California, United States. The event was the fourth time the UFC has hosted at the Honda Center (formerly Arrowhead Pond) in Anaheim, California following UFC 59, UFC 63 and UFC 76 and the sixth event held in the Greater Los Angeles Area along with UFC 60 and UFC 104. The event featured the most decision-based victories since UFC 64.

Just like UFC 111, UFC 115, and UFC 118, UFC 121 was shown in movie theaters around the United States by NCM Fathom. UFC 121 featured the return of preliminary fights live on Spike TV.UFC Primetime also returned to promote the Lesnar/Velasquez title fight.

Jon Madsen was expected to face Todd Duffee, but Duffee was forced off the card with a lingering knee injury and replaced by Gilbert Yvel.

Fighters were awarded $70,000 bonuses.

The following is the reported payout to the fighters as reported to the California State Athletic Commission. It does not include sponsor money or "locker room" bonuses often given by the UFC and also do not include the UFC's traditional "fight night" bonuses.

Brock Edward Lesnar (/ˈlɛznər/; born July 12, 1977) is an American professional wrestler, actor, former mixed martial artist and amateur wrestler, signed with WWE and appearing on its Raw brand. He is also a former UFC Heavyweight Champion and former #1 ranked mixed martial arts (MMA) Heavyweight by Sherdog; he was ranked #5 before he announced his retirement at the end of 2011. Lesnar is an accomplished amateur wrestler, winning the 2000 NCAA heavyweight wrestling championship and placing second in 1999, losing in the finals to 1999 world freestyle wrestling champion and future New England Patriots offensive lineman Stephen Neal.

He gained prominence in World Wrestling Entertainment (WWE) from 2002 to 2004, where he is a former three-time WWE Champion, becoming the youngest WWE Champion at age 25 with his first reign. Lesnar was the 2002 King of the Ring and the winner of the 2003 Royal Rumble. After leaving WWE, Lesnar pursued a career in the NFL. He played during the preseason for the Minnesota Vikings, but ended up being a late cut. Lesnar returned to professional wrestling at the end of 2005, and joined New Japan Pro Wrestling (NJPW), where he won the IWGP Heavyweight Championship in his first match. He was officially stripped of the title by NJPW in July 2006, and took the physical belt with him to Inoki Genome Federation (IGF), where he was still recognized as IWGP Champion until June 2007.

Cain Velasquez Ramirez (born July 28, 1982) is a Mexican American mixed martial artist fighting in the Ultimate Fighting Championship where he is a former UFC Heavyweight Champion. He is a two-time All-American collegiate wrestler from Arizona State University in Tempe, Arizona, and a Junior College National Champ at Iowa Central Community College. He is a two-time 5A state champion in Arizona, compiling a record of 110–10 for Kofa High School in Yuma, Arizona. Cain Velasquez is ranked #2 in the heavyweight division.

Velasquez, a Mexican American, was born on July 28, 1982 in Salinas, California to Efrain and Isabel Velasquez. Velasquez's father, Efrain, came from Mexico to the United States illegally. Efrain met his wife Isabel, married her, and gained US citizenship. Velasquez has two siblings, Efrain, Jr., and Adela. He was raised in Yuma, Arizona. He graduated from Kofa High School, where he compiled a record of 110–10 in four years of wrestling, including winning the 5A Arizona Wrestling Championship twice. Velasquez also played football for three years. As a senior, he served as Captain of both his wrestling and football teams.

Junior dos Santos Almeida (Brazilian Portuguese pronunciation: [ˈʒũnjoʁ dus ˈsɐ̃tus]), also known as Junior Cigano ("Gipsy"; born September 12, 1984), is a Brazilian professional mixed martial artist and the current UFC Heavyweight Champion. He is ranked as the number one heavyweight fighter in the world by numerous media outlets.

Dos Santos started training Brazilian Jiu-Jitsu under Yuri Carlton for fitness reasons. After some time he was invited to join an MMA practice, where he met his boxing coach Luiz Carlos Dórea, and after roughly a year he made his debut fight.

Dos Santos became professional in 2006 at the age of 21. He fought in small promotions in Brazil, such as Demo Fight, Extreme Fighting Championship, Minotauro Fights and Mo Team League. He won 6 out of his first 7 bouts, suffering his only loss to Joaquim Ferreira, a fighter he'd previously beaten.

Dos Santos debuted for the UFC on October 25, 2008 at UFC 90. Considered a major underdog, dos Santos knocked out top contender Fabricio Werdum at a minute and twenty-one seconds of the first round with a strong uppercut, earning himself a "Knockout of the Night" award.

Dana White (born July 28, 1969) is the current President of the Ultimate Fighting Championship (UFC), a mixed martial arts organization based in the United States.

Born in Manchester, Connecticut on July 28, 1969, White grew up in Las Vegas, Nevada, Ware, Massachusetts and Levant, Maine. White is a passionate Boston Red Sox fan. During his youth, he bounced back and forth between Boston and Maine. He attended the University of Massachusetts Boston but did not finish; however, while there he did launch a boxing program for inner-city youth.

White has a background as an aerobics instructor. In 1992, White established Dana White Enterprises in Las Vegas. He conducted aerobics classes at three gyms in the Las Vegas area and began managing MMA fighters Tito Ortiz and Chuck Liddell.

While working as a manager, White learned that Semaphore Entertainment Group, the parent company of the UFC, was looking for a buyer for the UFC. White contacted childhood friend Lorenzo Fertitta, an executive at Station Casinos, and a former commissioner of the Nevada State Athletic Commission. Within a month, Lorenzo and his older brother Frank bought the UFC, with White installed as its president. White currently owns about 9% of Zuffa, LLC, the entity the Fertitta brothers created to own and manage the UFC.