[Excerpt] Individuals began establishing health savings accounts (HSAs) in 2004. These savings accounts are generally used to pay for unreimbursed medical expenses on a tax- advantaged basis. Any unspent money accrues to the individual. To open an HSA, the individual must enroll in a qualifying high-deductible health plan (HDHP). HSAs are tax-advantaged and provide some incentives for people to monitor, and perhaps reduce, their expenditures on health care. Data covering enrollment and/or cost sharing during the first few years of HDHPs and their associated HSAs are now available from at least five separate sources. This primer provides information on the data sources, together with the most recent data available from each source on enrollment, premiums and deductibles.