Posen signed the band in 2001, reissued Identity Crisis, and put Thrice on tour with Samiam. Tours with Midtown and Hot Rod Circuit followed, and eventually Thrice reentered the studio with producer Brian McTernan. Although those recording sessions proved to be a trying period for the young group, The Illusion of Safety emerged and was later released in February 2002. Naturally, the band hit the road in support of the album, this time playing concerts alongside Further Seems Forever and Face to Face. Thrice also began headlining shows for the first time that year, and major labels began to take notice. Eventually, Island Records signed the band in June. A stint on the Warped Tour followed, and Thrice spent the fall playing club dates with Hot Water Music and Coheed & Cambria.
February 2003 found Thrice returning to the studio with McTernan, but this time Island Records was footing the bill. The focused effort The Artist in the Ambulance appeared in August 2003, and Thrice supported it with an ambitious slate of tour dates that included jaunts to Europe. The band also continued to involve itself with charitable organizations, having actively supported nonprofits and charities since signing with Sub City (the charitable arm of Hopeless Records). Portions of the proceeds from Artist in the Ambulance went to the Syrentha J. Salvo Endowment, which provided financial assistance for cancer screenings.
A new studio effort, the ambitious Vheissu, followed in October 2005, while the EP Red Sky appeared early the next year. A slew of new material arrived in 2007 and 2008, including a four-disc conceptual project entitled The Alchemy Index (issued in two double-disc releases: The Alchemy Index, Vols. I & II and The Alchemy Index, Vols. III & IV) and two separate releases of live material, The MySpace Transmissions and Live at the House of Blues. Meanwhile, the studio effort Beggars appeared in 2009.