Nurse practitioners are the principal group of advanced- practice nurses delivering primary care in the United States. We reviewed the current and projected nurse practitioner workforce, and we summarize the available evidence of their contributions to improving primary care and reducing more costly health resource use. We recommend that nurse practice acts—the state laws governing how nurses may practice—be standardized, that equivalent reimbursement be paid for comparable services regardless of practitioner, and that performance results be publicly reported to maximize the high-quality care that nurse practitioners provide.