Due to current or predicted health workforce shortages, policy makers worldwide are addressing issues of task allocation, skill mix, and role substitution. This article presents an example of this process in the United States (US). Health workforce analysts recommend that US physician workforce planning account for the impact of physician assistants (PAs) and nurse practitioners (NPs). We examined 40 state workforce assessments in order to identify best practices for including PAs/NPs. Most assessments (about 60%) did not include PAs/NPs in provider counts, workforce projections or recommendations. Only 35% enumerated PAs/NPs. Best practices included use of an accurate data source, such as state licensing data, and combined workforce planning for PAs, NPs, and physicians. Our findings suggest that interprofessional medical workforce planning is not the norm among the states in the US. The best practices that we identify may be instructive to states as they develop methods for assessing workforce adequacy. Our discussion of potential barriers to interprofessional workforce planning may be useful to policy makers worldwide as they confront issues related to professional boundaries and interprofessional workforce planning.