Institutional Special Needs Plans (I-SNPs) are a type of Medicare Advantage (MA) plan designed for long-term residents of nursing homes or those who live in the community but need a nursing home level of care. I-SNPs provide for on-site nurse practitioners in nursing homes to help improve primary care, care planning, and care coordination. Our study was the first to undertake a nationwide descriptive analysis of the I-SNP market. The share of long-stay nursing home residents enrolled in I-SNPs quadrupled from 2006 to 2021, from 2.2 percent to 8.8 percent. Despite this growth, nearly 70 percent of nursing homes did not have any residents enrolled in I-SNPs in 2021, and in more than 60 percent of US counties, there were no I-SNPs available. Nearly 94 percent of long-stay nursing home residents who were enrolled in I-SNPs in 2021 were eligible for both Medicare and Medicaid. More research is needed to understand why I-SNPs enter certain markets and why long-stay nursing home residents choose to enroll in them; to assess I-SNP enrollment barriers; and to compare hospital admission rates and quality outcomes among long-stay nursing home residents by I-SNP enrollment status. Increased understanding of I-SNPs could inform policies to address long-standing concerns about the quality of care in nursing homes.