Retail clinics have rapidly become a fixture of the US health care delivery landscape. We studied visits to retail clinics and found that they increased fourfold from 2007 to 2009, with an estimated 5.97 million retail clinic visits in 2009 alone. Compared with retail clinic patients in 2000-06, patients in 2007-09 were more likely to be age sixty-five or older (14.7 percent versus 7.5 percent). Preventive care-in particular, the influenza vaccine-was a larger component of care for patients at retail clinics in 2007-09, compared to patients in 2000-06 (47.5 percent versus 21.8 percent). Across all retail clinic visits, 44.4 percent in 2007-09 were on the weekend or during weekday hours when physician offices are typically closed. The rapid growth of retail clinics makes it clear that they are meeting a patient need. Convenience and after-hours accessibility are possible drivers of this growth. However, retail clinics make up a small share of overall visits in the outpatient setting, which include 117 million visits to emergency departments and 577 million visits to physician offices annually.