Physician assistants (PAs) and nurse practitioners (NPs) were introduced in the United States in 1967. As of 2006, there are 110 000 clinically active PAs and NPs (comprising approximately one sixth of the US medical workforce). Approximately 11 200 new PAs and NPs graduate each year. PAs and NPs are well distributed throughout primary care and specialty care and are more likely than physicians to practise in rural areas and where vulnerable populations exist. The productivity of NPs and PAs, based on traditional doctor services, is comparable, and the range of services approaches 90% of what primary care physicians provide. The education time is approximately half that of a medical doctor and entry into the workforce is less restrictive. The interprofessional skill mix provided by PAs and NPs may enhance medical care in comparison with that provided by a doctor alone.