Acting administrators usually assume the office in the interim period between the resignation of a previous administrator and the confirmation of his or her successor, including during the transition period between two presidential administrations, before the successor has been nominated and confirmed. Acting administrators come from within the EPA and usually hold an office that is subject to Senate confirmation before becoming the acting administrator. Linda Fisher and Stephen L. Johnson had served as Deputy Administrator when they became acting administrator.
Marianne Lamont Horinko was an assistant administrator at the time. They are not subject to Senate confirmation to serve as the acting administrator, though to continue to serve as a full-fledged administrator (as in the case of Lee M. Thomas or Stephen L. Johnson), they must be confirmed by the Senate. == Line of succession ==