As of June 2015, Aerosmith consists of Steven Tyler, Joe Perry, Tom Hamilton, Joey Kramer, and Brad Whitford. In the past, there have been other band members in the group.
Aerosmith was founded in 1970; Tyler (lead vocals), Perry (lead guitar), Hamilton (bass) and Kramer (drums) are all original band members. Ray Tabano was the band's original rhythm guitar player, but as a consequence of his eccentric behavior and mediocre guitar skills, he was replaced by Whitford in 1971.
In 1979, after personal disputes with other band members led Perry to leave the band, Jimmy Crespo took over as lead guitar player. Whitford left in 1981 to pursue a solo career; Rick Dufay replaced Whitford as rhythm guitarist. Dufay later encouraged Tyler to reconcile and reunite with his former band mates; in 1984, Perry and Whitford resumed their roles, and Crespo and Dufay left the group.
Occasionally, substitutes are used when band members are temporarily unable to perform. For example, David Hull has played bass with the band when Hamilton was recovering from various health issues. Other performers have performed with Aerosmith during concert tours, providing supplemental performances with keyboards or horns, but are not considered part of the band proper.