UFC 137: Penn vs. Diaz was a mixed martial arts event held by the Ultimate Fighting Championship on October 29, 2011 at Mandalay Bay Events Center in Las Vegas, Nevada.

The event was originally expected to take place on October 15, 2011 at the Echo Arena Liverpool in Liverpool, England. However, due to multiple scheduling issues with broadcast rights, the organization cancelled those plans.UFC 137 featured two preliminary fights live on Spike TV.

Dennis Siver was expected to face Sam Stout at this event, but Stout pulled out of the bout on August 29 and was replaced by Donald Cerrone.

Nick Diaz, who was originally scheduled to headline the card opposite Georges St-Pierre for the UFC Welterweight Championship, was pulled from the fight on September 7, due to failing to make media appearances. Carlos Condit was promoted from his fight with B.J. Penn to replace Diaz and face St-Pierre.

On September 8, B.J. Penn announced via Twitter that Dana White offered him a fight he "cannot refuse" and will still be fighting at UFC 137. Later that day, both Penn and White announced that the fight would be against Nick Diaz.

Nicholas Robert Diaz (born August 2, 1983) is a American mixed martial artist of Mexican descent. He is the former Strikeforce Welterweight Champion, WEC and IFC Welterweight Champion. He has competed in the Ultimate Fighting Championship, Pride Fighting Championships and Elite Xtreme Combat. Diaz was promoted to black belt in Brazilian Jiu-Jitsu by Cesar Gracie on May 8, 2007. Diaz trains at the Diaz brother's Cesar Gracie Jiu-Jitsu Academy in Lodi, California. Diaz holds the distinction of having never been submitted, and has been finished by strikes only once in 35 fights. He is currently ranked the #4 welterweight in the world by Sherdog, ESPN, MMAWeekly, and USA Today.

Diaz was born in Stockton , California. He is the older brother of UFC fighter Nate Diaz. He went to Tokay High School in Lodi for one year before dropping out. While a freshman, he was a member of the swimming team. Diaz has stated in the past that he is thankful his mother got him into swimming classes because swimming is one of the main reasons why his cardio is so impressive during his fights. Diaz has also said on multiple occasions that he had a hard childhood and grew up without his biological father.

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.