2000–2005: Formation and debut

In 2000, SM Entertainment held their first overseas casting auditions in Beijing, China and recruited Han Geng, who auditioned against three thousand applicants^. That same year, Leeteuk, Yesung, and Eunhyuk were recruited after auditioning for the company's annual casting system in Seoul. Sungmin and Donghae became trainees after jointly winning first place in an SM-sponsored contest in 2001. In 2002, Heechul and Kangin were recruited along with Kibum, who was discovered in Los Angeles, California by a casting agent. Siwon became a trainee after being scouted in 2003 and Ryeowook in 2004, who was recruited into the company after winning the 2004 Chin Chin Youth Festival singing competition^. Shindong and Kyuhyun, the latter winning third place at the 2005 Chin Chin Youth Festival, were cast in 2005.
In early 2005, Lee Soo-man announced that he had been preparing for an all-boy project group of twelve members to debut at the end of the year. He called this singing group to be "The Gateway to Stardom of Asia, as most of the members in this group were chosen for their experiences as actors, MCs, models, and radio hosts prior debut. Heechul and Kibum were already established actors at the time, and most of the other members had already made various kinds of appearances in television and media.^. Inspired by the rotational concept of Japan's girl group Morning Musume, Lee said that his new group would also experience line-up changes, with new members replacing selected members every year to keep the group constantly young and all-rounded. This concept was then newly introduced to the K-pop market.
For a while the group was rumored to be called O.V.E.R, the acronym for "Obey the Voice for Each Rhythm. However, before the group settled with their current name, the company simply referred to them as super junior05, a representation of the members' young ages when they first became SM trainees. After the members' showcased their different talents to the company at a picnic, the company finalized their group name to Super Junior, and officially became Super Junior 05, the first generation of Super Junior. Super Junior 05 made their pre-debut performance on the Korean channel Mnet on September 11, 2005.At the showcase, they performed various styles of hip-hop dancing, where they danced to B2K's "Take It to the Floor". Han Geng, Eunhyuk, and Donghae also performed a separate dance together, dancing to Usher's "Caught Up." However, the performance did not air on television until May 16, 2006 as a segment in the Super Junior Show, the group's first television documentary.
The group officially debuted on SBS's music program Popular Song on November 6, 2005, performing their first single "Twins (Knock Out)". A digital single with "Twins (Knock Out)", "You Are the One", and three additional tracks were released online on November 8, followed by the release of their debut album SuperJunior05 (Twins) on December 6, 2005. The album sold 28,536 copies in the first month of release and debuted at number three in the monthly chart of December 2005.