UFC 109: Relentless was a mixed martial arts event held by the Ultimate Fighting Championship (UFC) on February 6, 2010 at the Mandalay Bay Events Center in Las Vegas, Nevada.

A previously rumoured bout between Renzo Gracie and Matt Hughes did not take place on this card, but took place at UFC 112.

A Middleweight Championship bout between Anderson Silva and Vitor Belfort was confirmed by UFC President Dana White, although it was contingent on Silva's successful recovery from elbow surgery. Subsequently, the fight was rescheduled due to Silva's continued slow healing from surgery. The Silva vs. Belfort bout was expected to take place at UFC 112, however Belfort was forced to withdraw from the fight & Demian Maia replaced him, after number 1 contender Chael Sonnen was not clear to fight.

Antônio Rogério Nogueira had to withdraw from his bout against Brandon Vera due to an ankle injury. It was announced that Vera will face Jon Jones on March 21, 2010 at UFC Live: Vera vs. Jones.

Josh Koscheck was scheduled to face Paulo Thiago, but was forced off the card with an injury. Mike Swick stepped in as Koscheck's replacement and fought Thiago.

Chael Patrick Sonnen (pronounced /ˈtʃeɪl ˈsʌnən/; born April 3, 1977) is an American mixed martial artist who competes as a middleweight in the Ultimate Fighting Championship. He was an NCAA Division I All-American wrestler at the University of Oregon, a silver medalist at the 2000 Greco-Roman World University Championships, as well as a two-time Dave Schultz Memorial International Greco-Roman winner.

As of October 2011^[update], Sonnen is ranked as the No. 2 middleweight in the world by Sherdog.

Sonnen was born Chael Patrick Sonnen on April 3, 1977 in West Linn, Oregon.

Sonnen started his MMA career at the age of 19 in 1997 by defeating Ben Hailey in Vancouver, Washington. His second fight was against future ICON Sport Middleweight Champion and Strikeforce Middleweight contender Jason "Mayhem" Miller. Sonnen went 6–0 in his first six fights, until his loss to Trevor Prangley, which he later avenged in 2006 at UFC Ultimate Fight Night 4. He also fought The Ultimate Fighter winner and former UFC Light Heavyweight Champion Forrest Griffin.

Matthew John Serra (born June 2, 1974) is an American mixed martial artist and Brazilian Jiu-Jitsu black belt. He is a former UFC Welterweight Champion and an ADCC Submission Wrestling World Championship silver medalist. He began practicing martial arts at an early age, first studying kung fu. In the 1990s, he began studying Brazilian Jiu-Jitsu under Renzo Gracie and on May 23, 2000 he obtained his black belt, the first American to do so under Gracie. He is one of only two people in MMA to defeat Georges St-Pierre, the other being UFC Hall of Famer Matt Hughes.

Serra won first place at the Brazilian Jiu-Jitsu Pan American games in 1999 and won third place at the 1999 World Championships in Brazil as a brown belt. Continuing his Martial Arts career, he competed in the ADCC Submission Wrestling World Championship choking out Takanori Gomi, winning a decision over Jean-Jacques Machado, and placing 2nd in the 66–76 kg division. Serra was invited to compete in Pride 9 against Johil de Oliveira but the bout was called off at the last minute when Oliveira was burned in a pyrotechnics accident backstage. Soon after, Matt began to compete in the UFC organization where he built up a record of four wins and four losses. One of the losses was a close decision fight with future Welter and Lightweight champ B.J. Penn which would have earned him a title shot in the failed Lightweight tournament.

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.

Brian Michael Stann (born September 24, 1980) is an American mixed martial artist and U.S. Marine who competes as a middleweight in the Ultimate Fighting Championship. He is a former WEC Light Heavyweight champion. As of October 2011, Stann is ranked #7 middleweight fighter in the world by Sherdog.

Stann was born at Yokota Air Base in Japan and grew up in Scranton, Pennsylvania. He graduated from Scranton Preparatory School and then enrolled in the United States Naval Academy in 1999 as a member of the Class of 2003. He played football for the Midshipmen at the position of middle linebacker during his time at the Naval Academy. Upon graduation, he was assigned as an infantry officer in the United States Marine Corps, and rose to the rank of Captain.

On May 8, 2005, then Lieutenant Stann was assigned to 3rd Battalion, 2nd Marines and he commanded the 2nd Mobile Assault Platoon. During Operation Matador, his unit was ambushed by insurgents while trying to secure Ramana Bridge, near Karabilah. Stann and his Marines held out for six days under heavy attacks while Stann coordinated air and tank support that eventually allowed them to be relieved on May 14, 2005. All forty-two Marines in Stann's platoon survived, and the following March, Stann was awarded the Silver Star, the nation's third highest award for valor in combat. Part of the citation reads: "Second Lieutenant Stann personally directed two casualty operations, three vehicle recovery operations and multiple close air support missions under enemy small arms, machine gun and mortar fire in his 360-degree fight." This mission was shown on the episode, "Iraq's Most Wanted" on the History Channel show, Shootout!, in which Stann was interviewed. His actions were noted by President George W. Bush during one of his speeches. He left active duty in the Marine Corps as of May 2008. Captain Stann was also the commanding officer in the 8th Marine Regiment Headquarters Company while continuing to fight in the WEC.