After the success of SM Entertainment's boy band, Super Junior, the company planned to debut a new girl group, which would ultimately turn into Girls' Generation, featuring a start-up roster of 11 members. For nearly two years, Korean websites speculated about which of the female SM Entertainment trainees would be placed into the group. The candidate members were professionally trained primarily in singing and dancing, while some were also selected to be trained in acting and modeling. Many of the candidate members had already made names for themselves in the Korean entertainment industry through their participation in magazine adveritsements, television commercials, and small television drama or film appearances, and Sooyoung even debuted in the Japanese market in 2002 as part of the pop duo, Route θ, which soon disbanded in one year. The group witnessed several revisions, as members were either added or cut, until final line-up of the debuting nine was decided. Prior to their announcement, it was rumored that the group would be named "Super Girls" after their so-called male counterparts, Super Junior.
The first member of the group to join SM Entertainment's training system was Jessica in 2000, after her and her sister, Krystal Jung (now a member of the quintuplet girl group f(x)), were scouted in a mall in Korea during a family vacation.Members Sooyoung and Hyoyeon were also cast into SM's training system in 2000 through 2000 SM Open Audition, in which Hyoyeon danced for her audition. Yuri was the next Girls' Generation member to be accepted into SM's training system after finishing second in the 2001 SM Youth Best Dancer competition. Yoona was cast the following year after auditioning singing and dancing to her favorite singers, BoA and Britney Spears, in 2002 SM Saturday Open Casting Audition. Seohyun, the group's youngest member, was scouted in the subway by SM scout, then auditioned in 2003, singing children's songs. The group's leader, Taeyeon, was cast in 2004 after being discovered at SM's 8th Annual Best Contest winning "Best Singer 1st Place & Grand Award". That same year, member Tiffany auditioned at SM's Starlight Casting System in Los Angeles, and joined the company in October 2004. The ninth and final member of the group to be added was Sunny, who entered the system in 1998, training for five years before transferring to another company, Starworld, where she trained to debut in the duo "Sugar", which ultimately never debuted. Then under the recommendation by the singer Ayumi she re-transferred back to SM Entertainment in 2007. Sunny is also the niece of SM Entertainment's founder, Lee Soo Man.