Anti-lock brakes have been implemented on tractor-trailer units for several years. However, a fatal accident involving trailer swing indicated that there is some disagreement within the accident reconstruction industry as to what effects trailer anti-lock brake systems have on the stopping performance, dynamic performance/handling of the trailer, and resultant skid marks left on the roadway. Full-scale testing was conducted on a tractor-trailer unit which was equipped with anti-lock brakes on both the tractor and trailer. Full application braking tests were performed from 65-70 mph concurrent with a lane change. Baseline tests were conducted with all anti-lock systems operational, and the tire marks, amount of trailer swing, and stopping distance were recorded. The test was then repeated with the trailer anti-lock brakes disabled. All tests were videotaped from three positions: a stationary camera at the end of the test run, a stabilized camera mounted on a chase vehicle, and a stabilized camera mounted on the driver's side of the tractor cab facing aft. Results showed the types of marks left by tires both with and without anti-lock brakes. The tests also provided qualitative indications as to the effect of locked trailer tires on trailer swing. The measured stopping distance was also used to calculate an overall deceleration rate for the tractor-trailer, and this deceleration rate was then compared to speed-time data recovered from the tractor's engine control module.