The numbers of rotators—outside scientists, engineers, and educators on temporary assignment—at the National Science Foundation (NSF) and their costs in proportion to other staff remained relatively stable in fiscal years 2008 through 2017. Most rotators joined NSF under its Intergovernmental Personnel Act (IPA) mobility program. IPA rotators comprised about 12 percent of NSF's workforce and 17 percent of staff costs on average and were not subject to a federal salary cap. They remain employees of their home institutions, with NSF reimbursing the institutions for most of their salaries and benefits. The remaining rotators are considered temporary federal employees under the Visiting Scientist, Engineer, and Educator (VSEE) program; their salaries could not exceed the federal maximum for their positions.