This article explores how requiring employers to offer health insurance (an employer mandate) would affect health system performance along nine dimensions. An employer mandate would increase the number of people with coverage by 1.8 to 3.4 million; the newly insured would have better health, as measured by life expectancy. No studies directly analyze how an employer mandate would affect patient experience; it would have no effect on spending, consumer financial risk, waste, the reliability of receiving recommended care, or system capacity. An employer mandate would be moderately challenging to implement primarily because of the need to monitor and enforce compliance.