A review of nine studies of prophylactic knee braces worn by players in American tackle football lends some support for the use of double-hinged braces at the high-school level. Evidence for their use at the college level seems less persuasive. Caution should be exercised in interpreting these studies owing to the probable presence of bias and confounding variables and to difficulty in generalizing results beyond the study populations. The two studies that assigned braces randomly found lower injury rates for knees and knee ligaments among high-school and high-school size players. Conversely, a large, multiteam collegiate study found a significantly higher rate of knee injuries among brace users, a difference that remained when controlled for position, skill, and previous injury.